How chrome browser determine memory cache and disk cache?
Using this document, the Caching section in particular, I understand that there are two caches, an on-disk cache and a very fast in-memory cache.
As for your actual question, how does Chrome decide what gets cached where, I believe the next line provides the answer
The lifetime of an in-memory cache is attached to the lifetime of a render process, which roughly corresponds to a tab
While I am not entirely sure, I believe the in memory cache serves the current tab.