In the case of an image sequence, the rendering process creates the full frame in memory before the output phase begins for that frame.
Is there a specific reason that you're asking? Is there something that you're trying to do that depends on these details?
we're having a render farm problem with royal render...when we render 3d to the farm the server speed is fine, however when Ae renders the network and server read/write speeds slow to a crawl...25 nodes, figured after effects is writing every file to the farm as it renders...
Do you use the server also for feeding source material? What image formats? At 4k or higher, a bunch of uncompressed EXRs or whatever can easily eat up all bandwidth even on a normal gigabit network. Also if the server uses caching/ delayed write you may have a performance issue there...
TThe format is png sequences, and the server supplies everything, both reading from and writing to...but the nosedive the server takes when an ae render is kicked off...