is licesnce capacity based on usage or actually size or available size

I.e.
if i have 8 x 1 tb drives in a raid 5 that for some reason i only currently need 4tb of iscsi storage do i licence

a) - 4tb im using
b) - 8tb total size without raid
c) - 7tb of actual size

or as 8tb once raided and formated will come out at around 6tb i suppose thats an option to

thanks