I've always had a bit of trouble wrapping my mind around just what the Corporate Sector in the Star Wars universe was. I was never particularly satisfied with the Sourcebook that was produced for it by West End Games (primarily because the 'history' of the sector was boring to read- never a good sign). There were insinuations that most of the major corporations in the galaxy were somehow part of the Corporate Sector Authority, which just seemed odd to me, since many of the big Corporations were based OUTSIDE of the Corporate sector. Truth be told, I was always 'lazy' in my own setting about defining exactly what the Corporate sector was. That was until I received outside inspiration.
In the video game, Mass Effect, we are introduced to a planet called Noveria. It is a remote, high-security world owned 'jointly' by various corporations. This world does not answer to Galactic law, thus, the Corporations are free to do what they will, develop what they want, without regards to normal restrictions (pretty much anything is go as long as it doesn't affect the overall operations of the planet). Finally, I had some kind of grasp as to what the Corporate Sector was, or could be in my campaign at least.
The Corporate Sector Authority is essentially an entire sector that functions like Noveria. There is an overriding board of directors that manages the sector, provides security, services and all of that. Other corporations from around the Galaxy 'lease space' within the sector- whether it's a factory world or a mining planet or a research base, or a corporate high-rise somewhere. Here, free from the usual restrictions of the rest of the Galaxy, they do pretty much what they want, as long as it doesn't disrupt sector operations. In short, the CSA is a corporation that caters to other corporations. Maybe this was the original intent of the CSA in the WEG sourcebook, but if it was, it certainly wasn't stated clearly or succinctly enough for my short attention span.