This is simply assigning a range of channel to fixture (the range being the size required by the fixture).
There will be a fixture manager that will hold all the fixtures to be used. The fixture manager will be responsible for patching the fixture to channels.
You will be able to auto patch a fixture, this is where the framework decides which channels the fixture will use. In this mode you can query the fixture to see what address was assigned and prompt the user to set the base address on the physical fixture.
The other mode will allow the user to patch a fixture to a specific set of channels. This can be useful when a fixture is unable to be assigned to the entire range of addresses in a universe.
Each fixture can be patched in either way. The fixture manager will create the relevant soft patch objects, and then patch in via the channel manager.