Gamelan and improvisation. There is a section in a book "A Guide to the Gamelan" by Neil Sorrell about this:

"Because the word 'improvisation' has no absolute meaning it must always be used with care and myriad qualifications. To state that Gamelan music is improvised is likely to convey the impression of a freedom, even looseness, which it does not have; but to try to close the matter there would do the great disservice of denying it has that element of choice and interpretative spontaneity that is crucial to any great musical tradition.

"Garap, meaning treatment (literally 'working on') refers to the way a musician realizes another part from a given one...A similar situation would arise in our culture when one is harmonising a given melody at the keyboard or reading a figured bass. In each case the act involves improvisation, but a context is established and a framework within which the realization will be accepted but beyond which it will probably be judged unclear, inappropriate, and even wrong.

"No problem is thought to have but one solution, and garap is the art of finding the solution that fits best, but does not necessarily preclude others - nor the desirability of variation."

Which all seems very relevant to what happens in a good session, in our traditions.