Referencing Custom Objects from Add/Edit/CA Screens can only select one record

Upon referencing from custom objects -- whether through the OOTB screens (field layout > add/edit mode) or using a Custom Action -- it only allows selecting one record.

My use-case is a custom object of a "Product catalog" to add onto either the Program or Project level, so it can be more than one. Associating more than one after creation of the Program/Project is allowed, but not during the add/edit/CA screens; only a single selection/record from the Custom Object is allowed.