Datastore vs system disk quota

Anyone can explain to me the different between two quotas?
In my setup , I have 2 system datastores which are HDD & SSD and each datastore had 100GB quota.
I also set system disk quota 100GB.
However every VMs created (in SSD or HDD datastore) are only counted against system disk quota.
Is that normal behaviour? If yes then what is datastore quota?