I'm not sure what you're asking here, so I'll just describe a little of how REAPER moves data around for playback. Most of this is configurable, I'm just describing the defaults.
When REAPER reads from items on disk (i.e. WAV files etc), it buffers them ahead by 600ms (i.e. it tries to keep 600ms of data ready).
If you have the "fx render-ahead" mode on, REAPER will render each track (if it is not record monitoring or dependent on a track that is) up to 200ms ahead as well. This data is stored in a track cache.
Then, when the time comes to mix a buffer (the size determined by the audio hardware), the audio thread gets to just go mix down all of the pre-rendered tracks (or if they aren't already rendered, render them).
Is this helpful?