An order must always have a billing period and a price period. The billing period is how the invoice will be billed (invoiced).
Supported use cases for billing periods are Annual, bi-annual, quarterly, monthly, and end-of-term.
Supported use cases for price periods are Annual, quarterly, and monthly.
Price period is how your prices are communicated to customers. In any standard situation with annual recurring fees, the price period does not affect the billing. It becomes relevant to set if you are having any kind of used products.