Hello yeowkm99, It is best practice to select all space as usable.  Now with that being said you could leave some space as unallocated and use it for what you are wanting to do.
I have one set of VxRail system with 4 ESXi hosts, total VSAN storage space is around 120TB. currently its one single VSAN storage for all the VM in the 4 hosts. question, let say i can start from scratch to configure the VSAN storage, can i leave 30TB as unused partition for future use or I must by default select all 120TB as usable storage ? right now my usage is 25TB out of the 120TB with 85TB free space, which is think is rather wasteful
VxRail datastore storage