The API explorer shows something promising:
Apparently, each membership type can be connected to a 'domain' which is a contact ?
What exactly is it, and how could I assign these domains to a membership type ? The backend interface does not seem to mention it.
[EDIT] anything to do with https://docs.civicrm.org/sysadmin/en/latest/install/multi-site/ ?
On multisite you tie each membership type to a domain id. You can find what the domain ID's by looking at the civicrm_domain id table (or in the civicrm.setting.php file).
If you create the memberships on the correct domain they will be correct I believe, but if you don't then just adjust the domain_id in the civicrm_membership_type table.
Then only the memberships tied to a certain domain will be available for selection on a given domain.
