To start, we'll go to Memberships:

From here, we can EDIT an existing membership or create a new one.

I'm going to edit an existing one, the 2021 RENEWAL Membership. We'll go to Portal Settings tab:

From here, we'll scroll down.

Now we can customize if there are any memberships that users must have in order to register for this one.

This feature is particularly useful for clubs who provide membership on a year-to-year basis and want to offer a discounted membership type for renewing members.

I've selected the '2020 Membership' to be a prerequisite for this 2021 RENEWAL Membership.

Note: the 'ID' icon next to this membership. If you see this, it means that this particular membership type has a prerequisite.