First I would like to quickly explain about Buffer and Cache, so things can make sense up ahead :)
Difference between Buffer and Cache:

Buffer is a temporary location for storing data for a particular application and this data is not used by any other application.

Cache is a memory location to store frequently used data for faster access.

Both are temporary store for your data processing. However memory specified as buffer/cache can be made available when an application needs it, so we count it ...

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