There are two ways to configure memberships so that development revenue is either recorded as part of the membership dues (Dues Based program) or memberships can be awarded to constituents once a specific type(s) of revenue is given up to a specific threshold (Contribution based)

Contribution Based memberships must be configured from the beginning.  There isn't a way to enact a Contribution based program after the fact.  Rules for what types of revenue are evaluated are defined on the Contributions tab of the memberships program.

Dues based memberships can be tracked as pure revenue source where 100% of the membership payment is attributed toward membership in reports and in the General Ledger.  Dues based programs (or levels in the case of both ways programs) can also have their revenue split into Designations so that a portion of the membership payment is counted as a donation and the rest is tracked as membership revenue. Rules for what Designations are used are configured on the Dues tab.