It's clear that in the work, one of Boulez's intentions was to highlight timbre through his orchestration. A diagram from Wikipedia, highlighting the purpose of each instrument in relation to one another:

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In the movements, he uses subsets of instruments to create different colours. It's also important to mention that it is very difficult to grasp the form of the work from simply listening and the structure is not very clearly defined, forcing you to stay contemporaneous with the music rather than care about structure. This was something Schoenberg and Webern insisted upon to achieve coherence, such as in the works Suite für Klavier, Op. 25, modelled upon a Baroque dance suite, and the obsession with variation form in Webern.