First, you will want to add the User Detail object. This can be found under Select Objects, by expanding Constituent Information.

user detail under constituent information

In the Filters tab, you will need to set User Detail.Boardng or Day to "starts with" and type either B or D depending on if you are filtering for Boarding or Day Students.Then, click Select to save.
For example:

User-added image
In the above example, the list will be filtering for Boarding.