Relation rules
Cardinality, applicability and uniqueness of a relation, and what Explorer then refuses.
Relation rules
Targets per source is how many target records one source record may have; sources per target is how many source records one target record may have. A blank value takes the relation type's default. Explorer refuses a link past a maximum and an unlink below a minimum, naming the rule.
Discriminator fields (under Rules & settings): when several links per pair are allowed, two links between the same two records count as different slots if they differ on every field ticked here, for example two roles in the same department. Only the relation type's own fields can be chosen. With one link per pair the choice is kept but not consulted, and the screen says so. Save with Save discriminators; saving with nothing ticked clears them.
Selected source entities / Selected target entities: when a side applies to entities chosen one by one, tick which it accepts: the side's own entity or entities that inherit from it (set under Inherits from). At least one must be ticked. Otherwise the screen says no selection is used; change the side in the settings and save first.
Keep in mind Relations and fields that belong to the Platform or the Account are shown with their rules, which can be changed only in that scope.