Why don't you contact one of the SP's who have this in there add, act like your potential client and ask them.
Many providers these days are requesting "no explicit talk" over the phone because off course the point is to get you in the door.
After which because of thinking with your little head, you are at their mercy whether they wish to scam you,offer you much less than was promised in their ad etc.
Many times it's someone other than the provider you're speaking to on the phone, who will in turn promise you anything to get you in the door.
So best to visit personally on your least horniest day after jerking off before hand for good measure, and inquiring with a straight face.
Can't find any of the adds right now,, have been looking... LOL
West Drive has it in their Ad everyday in the Massage advertising section here.
Probably the only place I remember seeing the phrase.
That "S" is for ambiguity and to stimulate your imagination to run wild while being discreet. It can mean anything depending on the provider you meet,how weak or strong you are perceived as a client, and his negotiating skills, how much he is willing to pay for what he wants "S" to mean and how responsive the provider is to his wallet.