Under Core > Settings > Relationship Types, any Relationship Type with the "Parental Access" checkbox available (even if not enabled) is considered a parent/child type relationship.

This includes Parent/Aunt/Uncle/Grandparent/Guardian/Consultant/etc.

The list looks at all users with those Relationship Types to the child, then they are ordered #1 through #x, based on who was entered first into the database. It is not looking at just the Relationship Type.

For example, if a student has a Grandparent that is an Alumni, they'll likely show under Parent 1 if they were the first of the users added into the database.

It looks at which user was added into the database first, and if they have a Parental Access-type relationship with the student.

To filter only by a specific Role such as Parent, you will want to add the User Role Object for the Parent User Base, also Add a Filter of  the User Role.Role of Parent.

For more information reference List with Parent/Child Relationships only Parents listed in list