The supertype table Persons has several "child" tables containing information (e.g., contact information) that applies to all "persons" -- both employees and non-employees.
However, subtype table Employees has several "child" tables (e.g., EmployeeTime, EmployeeCertifications, EmployeeTraining, etc.) When creating views, etc. which include these tables, it seems more straightforward/self-documenting to join on "EmployeeID" vs. "PersonID", since the join is on the Employees table and NOT the Persons table.
Does this make sense?