What does this section cover?

When to use 'Submit Buy in Advance Request"' 

Submit Buy In Advance Request action should be used in cases where the subscriber agrees to be billed for a longer period of time than the specified billing frequency in exchange of a discounted price.

Submit Buy in Advance action differs from the rest of the Subscriptions action types, in the way that it can be requested. It does not have a behavior and business code as the rest of the actions, and when submitted a request is created that is handled by the Billing Engine during the Billing Run, rather than an actual Subscription action

For a complete description of how to use the specific action go to Creating Buy in Advance Requests