I must be naïve but I do not know hat that stands for

Google is perfect for answering this kind of question on your own. I googled "YMMV" just now and got over 3.5 million hits. I could tell you what "YMMV" stands for, but first you should try using Google and see what you get.

Hey there. The answer to this and about a thousand other questions can be found in the instruction manual that's stuck to the top of the newbie board. You should read it. Here's a link to a small slice of the information there, where you'll find some sense in the alphabet soup.

is what it really means.

 
On that point, there can be some differing opinions.    

 
On the surface, it means certain things a gal will do with some guys, she won't do with others.   The classic case is greek where a gal might let someone with a smaller penis do this but shut out a guy with a large one.

 
Then there are a host of other things that may depend on anything from your looks and/or age to hygiene and appearance.  Even manner can affect whether or not a gal will let you, say, massage her or not.

 
I'd say that it is easily the hobby's most confusing acronym.

Really Fish? The most confusing? Huh. I guess to each their own, it's surprising how different people's experiences can be, and all that. I'm sorry, but even when I was brand new YMMV made perfect sense to me.  

Even though she's being paid, a woman is not going to be ready, willing and able to do exactly ALL the same things in bed, every day, with any swinging dick who happened to find her online. Where's the confusion? This is their business, yes, but it's an extremely intimate business and not all clients are going to be viewed in the same light. TBH, I don't see what's confusing or surprising about that. To me, it makes perfect sense to anybody who ever met a woman. That's just me, I guess. It may not be "fair" but it makes sense.

I will say that what can be difficult to understand is exactly which factors come into play; how she makes her decisions regarding what to do or not do and with whom. That's a woman being confounding, not the acronym LoL!

IMO, the most confusing acronym, or at least the most controversial, would be GFE.

why some providers will do "X" with John1 and not do "X" with John2.  

I agree, except with the last statement. But your statement on YMMV very accurately portrays the ideals of what the providers believe it to mean (that I have communicated with about it, and some friends).

From my own experience, it's been a matter of trust and the repore.  

Ooohh now you've got me curious. What do you see as the most confusing/controversial term? Just curious.

the proper way to treat a lady.

GFE has always meant different things to different people. Other acronyms can also be confusing. However, the meaning of YMMV should be very clear. This is not the only "industry" that uses that term either.

on the guy, how CLEAN is, how it smell, and many other factors.  

The same thing I can do with one client.. I can with others. Mainly the issue is usually strong body odor... or any time of odor lol

"The Grelling–Nelson paradox is a semantic self-referential paradox concerning the applicability to itself of the word "heterological", meaning "inapplicable to itself." It was formulated in 1908 by Kurt Grelling and Leonard Nelson and sometimes mistakenly attributed to the German philosopher and mathematician Hermann Weyl.[1] It is thus occasionally called Weyl's paradox as well as Grelling's paradox. It is closely analogous to several other well-known paradoxes, in particular the barber paradox and Russell's paradox.
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Suppose one interprets adjectives "autological" and "heterological" as follows:
An adjective is autological (sometimes homological) if it describes itself. For example, the English word "English" is autological, as are "unhyphenated" and "pentasyllabic".
An adjective is heterological if it does not describe itself. Hence "long" is a heterological word (because it is not a long word), as are "hyphenated" and "monosyllabic"." - Wikipedia
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Is YMMV a term whose meaning is YMMV? Then it cannot be YMMV.  
If the meaning of the term YMMV is NOT YMMV then it must be YMMV.
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I think I need a couple of hours with the Whore of Mensa.

They look at the past reviews and her profile and think that they are ordering a set of services from a menu.  It doesn't work like that.  Even with the same lady, the experience could be different each time.  I just kind of let thing happen on their own and don't have a set expectation.  If I treat her right, she'll do the same.  It's worked well for me.

Many people reading this site might not be US-born and raised and not familiar with the USA culture and idioms.  Where did the "YMMV" "Your mileage may vary" phrase come from and come to have its current meaning?
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At some point, in the 1970s I think, the US Government mandated that car companies provide information to consumers about the fuel efficiency of their cars. Run a few tests on a test track or stationary car treadmill and include the info on the car sticker. "35 MPG" meant that the consumer could expect to get around 35 MPG from that car.  
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People started to complain to consumer protection agencies because they weren't getting anything CLOSE to the claimed fuel efficiencies. Since when does 35 MPG = 19 MPG? So the car companies had to explain things along the lines of, "We tested that model and we got 35 MPG, but, of course, YOUR MILEAGE MAY VARY."  After all, you are not a professional driver, driving a perfectly tuned car on a clean, smooth flat track in Yuma, Arizona.  You are a lead-foot driver with 1200 pounds of passengers and extra  junk in the trunk (double entendre intended) driving on pot-holed streets in real traffic.
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"We are not guilty of false advertising a high fuel efficiency because we are telling you that YOUR MILEAGE MAY VARY. Even if it is in tiny print."  
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More rules and regulations (highway MPG; city MPG; standardized testing so you can compare a Chevy to a Ford and hope for the best) followed.  
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But YMMV entered the language as a way to evade accusations of false advertising or providing misleading info in such user-dependent matters.  
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Can you get 35 MPG? Sure, but in reality, YMMV.
Can you get BBBJCIM? Sure, but in reality, YMMV.
Can you get MSOG? Sure, but in reality, YMMV.
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From a Provider's perspective,
Can you get 35 MPG? Sure, but in reality, YMMV.
Can you get BBBJCIM? Sure, but in reality, YMMV because you are not all the Perfect Test Driver who arrives clean, acts politely, knows how to rev my engine, etc.
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YMMV is not a protection against FALSE advertising.  
Claiming that "Our beauty cream contains aloe." when it does not, is not a YMMV claim. It is a lie.
Claiming that "BBBJCIM is on the table." when it is not and never was is not YMMV, it is a lie.

-- Modified on 10/3/2017 8:10:32 PM

In my personal experience GFE men prefer an intimate experience with kissing, cuddling, even pulling down the bed sheets!  Definitely a far cry from the man who DFK (deep French kisses) me the second he walks in the room (which is also fine by me!).  Missionary position preferred as opposed to others from the back so they can see your face and they like to be caressed, massaged, and talked to in a more sophisticated way.  My GFE gentlemen generally do not want toys placed inside them nor eagerly perform anal rimming.  I can't speak for others, but most people probably do a little GFE and PSE (porn star experience) in each appointment as it is most like boyfriend/girlfriend relationships.  

I too Google or watch porn to learn!  Some of the adult shops offer classes too!