Main parents family in Gramps
The concept of main parents family is useful in Genealogy, and therefore in Gramps you can define one of the families that a person is affiliated to to be the main parents family for that person. Whenever Gramps has to decide which parents family is the most important for a person, for reports, etc, it will decide based on this. The main parents family is the first family in the parents family order. The order can be changed by using the Reorder button in Relationships View.
Cf. src/gen/lib/person.py, get_main_parents_family_handle().