The System Picker

In the Rebuilder/Scanner/Merger you have a button called "Systems...". After clicking on that, you'll get the following (resizeable) window:

 

The listed systems can vary. They depend on the datfile you're using. Take a look at possibly existing "resource" entries in the datfiles. These are the used systems. Normally these are systems which share a BIOS.

There is always at least one system called "Standard". This standard system belongs to all romsets which aren't one of the other listed systems.