The Contact ID (used to uniquely identify Contacts for each Customer) default setting defines the Contact ID to be used when a Customer's first Contact is created.
The Default Contact ID is automatically assigned to the first Contact created for each Customer and cannot be changed.
Default: left blank
If the Use when Adding First Contact parameter is enabled, Keystroke automatically assigns the Default Contact ID to the first Contact created for each Customer.
Note: When the Use when Adding First Contact parameter is enabled, it is best to leave the Default Contact ID blank. The default Contact is the first Contact added to a Customer, has a blank Cont ID, and is automatically added to Sales Transactions, typically to represent a shipping address that is different from the Customer's mailing address and is used for printing on invoices and receipts.
See:
Company Defaults Settings
Configuration Manager Contents