Thank you Chris and Kitz.
This all came about because following a BT fault Plusnet were quoting various jargon at me, without any explanation as to what it meant.
At other times it was obvious they were quoting terms which meant something totally different to me.
It would be nice if with subjects like Data Rate and other terminology there could be a precise definition clearly spelled out which everybody adhered to.
Perhaps I've been looking at this in anticipation of a simple clear cut answer when the issues involved are inevitably complex, and don't necessarily lend themselves to such a simplistic approach.
I'll continue to digest what Kitz has kindly written and once again it is much appreciated.
Edit: I cringe when someone begins a Post 'Just a little query'