What is the purpose of the cache tier in an all-flash vSAN configuration?

What is the purpose of the cache tier in an all-flash vSAN configuration?
A. The cache tier is used only for write buffering.
B. The cache tier is used only for read cache.
C. 70% of the cache tier is used for read cache and 30% is used for write buffering.
D. 50% of the cache tier is used for read cache and 50% is used for write buffering.