A320 yellow accumulator
 

FCOM says there is a system accumulator in each hydraulic line downstream the pump to cover the transient demands.
Is the yellow system accumulator the one used for parking brakes or emergency braking or 'braking accumulator' is separate from the system accumulator?

It's a separate braking accumulator.

G+Y system low pressure or dual eng failure, applying brakes can be accomplished by this brakes accumulator located as the starboard MLG bay aft frame. you could simply see this on a walk through or while parked.

applying brakes are limited to 7 applications and upto 1,000psi only