hi Tobias,
Sub-fixtures is a bit of a difficult concept, but once you crasped it I hope you'd find it a very natural way of thinking about dmx-fixtures that house multiple fixtures inside.
this is a text from cuelux.com:
Sub-fixturesCuelux uses the concept of sub-fixtures. This means that some fixtures consist of multiple sub-fixtures, like a 10-channel dimmer rack is - in our terms - just one fixtures with 10 sub-fixtures. This also works for LED fixtures with multiple segments/pixels. In this manner the maximum of 24 will not be reached that quickly (you can add 24 x 10 = 240 dimmer channels). The reason behind the concept of sub-fixtures is that it makes patching easier; you only add the physical fixtures to your patch and you don’t have to worry about setting starting addresses per dimmer channel, or worse, per LED segment.
hope it helps to clear it up....
So you can only patch fixtures - any unit you've patched is thus a fixture. Some Fixtures contain sub-fixtures (like dimmers, pixel bars and the DJ-LED QCF-400).