Surrogate key in Axapta

A surrogate key is an artificial value that has no meaning to the user, but is guaranteed to be unique by the database itself. 

Surrogate key is like an Index defined by the application.And RecId is automatically populated by ax for reference purpose in table level.If you didn't specify any primary index then system will specify one default index called surrogate key.

If You did not defined any index for the table directly take recid as surrogate key.