When your a regimental Signal Instructor or so it used to be. RSigs doesn't wear them. In the Infantry you used to wear the left arm at the bottom if you'd done your class 2, right arm if you were class 1 (Regt Sigs Junior) and above tapes if your Regt Signals Det Comd (or the old Regt Sigs Instructor/new Regt Sigs Senior). Something similar exists for most of the other corps.
Used to be same for mortar quals, numbers wore them on the left arm and those that had done the Warminster course wore them above tapes.
I think I'm still qualified to wear both but I don't bother (as it make look a little waltish). The wearing of badges on lower arms doesn't now happen with C95/combats but should still be worn on Number 2's, jumpers etc
In the infantry your det comanders taught you how to teach (although was called Regt Sigs Det Comd - now I think its called something else and the content may have changed), it was far more in depth than the BITS type courses, also contained quite a bit of management).
We don't have an equivalent, its more like our Class 1/supervisor courses with orders and BITS added to it