Some is this styems from the US system of paperwork.
Unit - used to hang your lights on a pipe. Unit 1 being the first instrument on a pipe (as seen from Stage Left in the US), Unit 2, the second, etc.
Channel - The number by which a fixture is identified on the lighting console
Groups - used by capture to indicate a group of lights that you want to easily selct together
Circuit - the socket number to which a fixture is connected (could be a multicore, in which case you could get a letter and a number separated by a dot)