Seeking insights in The Trainable Cat
by Bradshaw and Ellis, I found this:
As an illustration of just how flexible their behavior is [in their third month], one of the skills that young cats learn during this period is how to get our attention, specifically by meowing. This socialization is so characteristic of cats that it is often presumed to be instinctive. However, feral cat colonies are generally rather silent places, the cats communicating mainly by body language and scent, and not vocally: cats generally reserve this sound for communicating with humans. Young kittens instinctively meow to their mother to attract her attention, but she stops responding when she wants to wean them, and when they discover that calling out to her no longer provokes a reaction, they stop doing it. However, the meow, though dormant, evidently remains in their repertoire, and as they adapt to their new home, cats find that meowing has become effective once more, this time in attracting the attention of our own species.
The book goes on to say that meowing becomes an idiolect, with meows for different purposes understandable by each cat and owner but meaningless to anyone else.
I can testify to the motivation in which "every cat works out for himself which meow produces the desired result," because I could hear our former cat Pandora trying it out on the drive to the vet. She would meow constantly, using a variety of pitches, lengths, and other variables, in evident hope of hitting upon the one that would persuade us to turn around and take her home.
Maia, however, didn't begin to meow until fully into adulthood, and has no special code. She comes into my office and meows in the same general way whenever she wants something, and since she only ever wants three things - food, petting (actually scritching and nuzzling), or playing with a peacock feather - I determine which she wants by where she goes on leading me out of the office.
If she wants food, she goes to the hall bathroom, where her dish is kept. If she wants petting she goes to the bedroom, because she's learned that petting only takes place on top of the bed, where cat and I can most easily be on the same level. If she wants playing, she flops down on the hall landing, which is cue for me to go halfway downstairs, fetch the feather from on top of a downstairs bookcase, and waggle it at her through the railing. Maia is the laziest huntress in the feline kingdom, her method of chasing the feather consisting of lying on the floor on her back and wiggling all four paws at it.