What is " This Reservation Reserved " parameter ???

Hi All,

  I have been asked to set the parameter in VCAC " This Reservation Reserved " while adding the Data store resources in VCAC 5.2. I dont have much knowladge on VCAC , I dont want to configure some thing wrong.

Any one help me to understand what is this parameter, how to calculate ??

For Example:

I want to add Adding 2 TB data store and limit the usage to 80 %. what i should configure " This Reservation Reserved "

Thanks in advance !

-Ravi