I encountered this in the context ben sost. which I took to mean "ben sostenuto". I've also seen it in ben marcato and ben attaca as well as in many other directives.
Little did I know, ben is an abbreviation for bene, which means "good" or "well". But something like ben sostenuto doesn't mean "play good and sustained", it means "play well-sustained". What's the difference? ben sostenuto means to play the best sustained you can manage -- if someone were to look up sostenuto in their musical dictionary, there would be a recording of you playing this ben sostenuto passage.
I must emphasize, however, that this does not mean that you can play "mediocre sostenuto" whenever you don't see a ben next to your sostenuto, or whatever annotation you're currently working with!