I think my question was more - is it a 'unit' that is separate, geographically, from a mainstream school, or is it a class which is part of a mainstream school.
As I said, language and labels are different across the country.
The last time I hear 'unit' used in my authority was about 20 years ago, and it would have been a class, within (or part of or attached to) a mainstream school. In my authority, these have now morphed into what they call Resource Bases, and the aim is that the children that go there will gradually spend more and more time each week, and each day with their mainstream peers and gradually reduce the time in the RB. When they start, they will spend most of their time in the RB Classroom and then, as suits the individual, will spend more and more time with their mainstream peers. In some schools they start the day (registration, assembly etc) in with their mainstream peers and then go to the RB for specific lessons. In others, their 'home room' as it were is the RB and they gradually start attending lessons with their mainstream peers.
This ^ is what I would have assumed a 'unit' was.
However, from your second post it sounds more like it is a separate place, not part of a mainstream school, so, in effect a special school ?
I think they are quite different approaches, which is why I was trying to clarify what your authority mean by 'unit'.