Primary line IS THE shared datapool.
Each supplementary 48 comes with 20GB split; 10GB is for its unshared exclusive use, whilst another 10GB goes into the shared datapool.
Illustration for an account with 128 primary and 2× supplementary:
Primary = Shared datapool = Primary's 40GB + Supplementary's shared 10GB + Supplementary's shared 10GB = 60GB
Supplementary #1 = exclusive 10GB
Supplementary #2 = exclusive 10GB
Supplementaries will draw from its own private allocation before it can draw from the shared datapool.
Primary line have authority to limit sharing to each individual line between 100MB to unlimited.
Illustration for consumption limit
Primary = always unlimited since IT IS THE shared datapool
Supplementary #1 = own 10GB + max "5GB" from shared datapool
Supplementary #2 = own 10GB + max "100MB" from shared datapool (practically almost disabled from drawing)