Synonym domains are special domains that are reserved by the system. You cannot add new synonym domains, but you can add new synonym values that are presented to the user.
An example of a synonym domain is work order status. The system has several values to reflect status: APPR (Approved), CAN (Canceled), CLOSE (Closed), COMP (Completed), WAPPR (Waiting on Approval), and others. Each work order status has an internal value, used by the system in its business rules, and a value that users see and choose from. You cannot add a new internal value. You can add a synonym, the value that is presented to the user.
Suppose your company procedure requires two people to approve a work order. You could add synonym values for the internal WAPPR value. You could then present two different values to the user, for example, WAPPRMAN and WAPPRVP, to represent approvals at the manager and vice president level.