For example, -r102 will choose Color BASIC 1.2, Extended Color BASIC 1.0 and Disk BASIC 1.1. Generally there's little need to use anything but the default latest versions of each ROM but it can be very useful when testing program compatibility or investigating the ROMs themselves.
Color BASIC has 4 versions numbered 0 .. 3: 1.0, 1.1, 1.2 and 1.3. Extended Color BASIC and Disk BASIC only have two versions each, 1.0 and 1.1.