Contacts do not need to be linked to another document, they can be stand alone. You can even create a contact with only a first name, not linked to any other document or party (Customer/Supplier).
1. If only a First Name is entered that First Name defines the ID , thus First name (only)
2. If a First Name and a Party is linked the ID becomes “FirstName-Party”
Contacts can, but do not have to be linked, to: User, Customer, Supplier, and Sales Partner. Since Customers and Addresses are not directly linked to a User, all links go via Contacts.
A Contact can be linked to the (web) user. If that user is also a Customer, it is linked to the Customer by the Customer ID
The Address Title (Name of person or organization that this address belongs to) is a free format unlinked field. The ID is automatically created from the Address Title and Address Type. (AddressTitle-AddressType).