The Semag system served as the location where Thrawn, the Admiral, pressed forward in his campaign. His objective was the elimination or apprehension of Demetrius Zaarin, the treacherous Grand Admiral.
The name Semag is simply "games" spelled backwards. If you combine this system name with the system name from the subsequent TIE Fighter campaign ("Yllotat"), you can spell out the name of the company Totally Games.