Difference Between the Filter Customer (Primary) Fields and Customer Fields on Contact Saved Search

Answer/Solution: 

1. Navigate to List > Search > Saved Searches > New >  Contact

2. Under Criteria Tab, notice that there are 2 customer fields.

      Customer (Primary) Fields..
      Customer Fields...

Customer (Primary) fields  filters the fields for primary contacts only while the other Customer fields  is used to filter fields for all contacts.