A principal member is usually a substantive member with related records also in the Member Database.
Each member has a Principal Member field. You link e.g. a family member to their Principal Member by typing a few letters of the Principal Member’s surname into their family member’s record and pick from the list. This is useful for connecting family members together. The Principal Member can click through to see details of all their related entries, and they can edit some fields in those entries if you turn on EDIT_FAMILY via Site Config.
Note the Principal Member has nothing in the Principal Member field of their member database record.
Principal Member relationships are often used when we code custom rules for you. You can also turn on options to consolidate subscription or booking invoices to the Principal Member, using Configuration Items:
If you turn one of these on, it operates for all Principal Member relationships that exist in your member database.