When you spend millions and millions of dollars on a new Motorola trunking system, one of the things you get, besides some t-shirts and mugs, is a floppy disk with the system key on it. The system key is a small file that allows you to edit trunking parameters in the RSS software. Say I wanted to program up my new MTS2000 for system 1234. I would need the system key in order to create that system in the RSS. Then, if you want to edit things like the radio ID, talkgroups, or control channels, you will once again need the system to change these parameters. If you don't have the system key, then you can't create a trunk system for that system, nor can you edit any parameters. It is kind of like the key for Windows of Office or what not. If you don't have it, then you don't play.
If you want to buy a trunking radio off like eBay and use it to monitor your local police, then you will need, besides the RIB and software, the system key,
There are ways to create your own system key....
Jeff