What is the difference between a Primary Line and a Secondary Line?

The Primary Line manages and pays the monthly cost for the entire family. The Primary Line can also add or remove Secondary Line(s) from the Family Account. The Primary is responsible for managing the family and making sure that the renewals are paid on time so that every line in the family can remain active.
The Secondary Line can remove themself from the Family Account, and the Secondary Line can also pay their renewal cost every month if they so choose. The Primary is ultimately responsible to pay the outstanding renewal balance each month, but each Secondary line can theoretically pay their own monthly cost if they want to, but they can ONLY pay for their own line.