When a new contact is added to your database, they are automatically assigned a unique number called User ID (aka Member ID). This number is generated by the system and cannot be modified (even via import). It will continue to be used to identify the contact until the contact record is deleted.
When importing contacts in bulk from a spreadsheet, Wild Apricot will use the User ID (or email) to match the records being imported with existing contacts. If the User ID already exists, the existing contact record will be updated with the information being imported. Otherwise, a new record is created.
Administrators can view the User ID from the contact details screen.
The User ID appears to contacts as the Member ID on their member profile.
Administrators can prevent the Member ID from appearing on the member profile by checking the For administrator access only option for the User ID field on the Common fields screen.