VNX Array - Adding a volume that passes a user defined limit

I am working on an older VNX Array that has a datastore for VMware which is out of capacity. There are two pools created named 'iSCSI Pool' (SAS Disks) and 'CIFS Pool' (NL SAS). The iSCSI pool is at capacity but there are 1.6TB free on the CIFS pool. This naming that was done I believe is simply to group the disks together for their use cases. In this case I would be taking a host using iSCSI and adding a new storage datastore that sits on CIFS pool. I don't believe this matters at all but perhaps is a waste of the NL SAS disks. 

My concern is that if I increase any capacity it will push past the 75% allocation and to 90%. What is the purpose of this overhead? Is it best practice to leave that certain percentage available or is it more of a warning threshold?