The value that you put in the field USER_MASTER.Name is the value that is shown in the applications to identify the Outsystems User (this is the value edited in Enterprise Manager). There is no rule stating that it must be directly related with the FirstName and LastName fields (that, of course, normally store the first and last name of the User). In fact, many times the FirstName and LastName fields are not even used.
To give you an example, I worked in a customer where there was a synchronization of the outsystems USER_MASTER table with the Domain Users in the Active Directory and the Resources in the HR System. The rules of the synchronization were the following:
I hope this info was helpful,