From this Chapter:
We have seen that in the chamber music works considered here, Xenakis is denied the inevitable masses of a traditional orchestra, ensembles comprised of up of 4 to 6 times as many individual musicians than there are in these chamber ensembles. As a result, he forms small characteristic units and gives them very specific, archetypical tasks: act together, act independently; act regularly, act irregularly; go up, go down; go down occasionally while going up, and vice versa; gradually slow the process you are carrying forward, or the reverse; do what you are doing continuously, do what you are doing discontinuously. This list could be extended, but what it already demonstrates is that Xenakis knows how to use his version of musical materials in ways that are fully as varied and eventful as more traditionally structured musical motives and themes.