When a contribution page is configured with a Membership section enabled and the "Separate Membership payments" enabled, transactions for the membership and a separate contribution will be processed separately and both categorized as the "Contribution type" specified on the main contribution page settings.
It would be very useful for accounting purposes to allow the separate payments to be put into two categories. (e.g. Membership payment -> "Membership dues", and an additional contribution -> "Contributions").
When "Separate Membership payments" is selected then an additional "Contribution Type" drop down should allow the membership transaction to be categorized separately.