Let's see if I can shed some darkness and corfusion on your clearly-put questions.
"Some of the letters I understand but, what dose BG mean ?"
BG is "blown gas", just like Rich guessed/opined.
"If A is altered and M is modified, what are they referring to ?"
A simple way of explaining is that "modified" allows some changes, "altered" allows more radical modifications. An altered bike MUST meet at least one of four or five requirements, such as foot pegs no more than 6" in front of the rear axle, or a fuel tank of LESS THAN 1.32 gallons capacity, or a riders seat that's BELOW the top line of the rear tire, a wheelbase at least 110% of stock, and maybe some others. Get the rulebook and you'll find 'em all.
"If I run nitrous, would I be in the F class ?"
Yes.
"It runs on gas/petrol but is injected with nitrous oxide and the swingarm has been extended.
What class would I run in ?"
Well, since I've already mentioned that there are some things, such as a swingarm that's over a certain length, that would put you in altered -- and you don't specify the length of your extension -- I can't guess if you'd be in modified or altered. But for sure you'd be in fuel class -- the nitrous puts you there.
"What distance are your records timed over and are they two way averages ?
How much run in and stopping area is there ?"
There's a one-mile (approx) run up, a 132' timing trap, and a shut down with first turnout at about 1/2mile, the long one at something like 3/4 mile.
Now there -- that wasn't so bad, was it? Look at some of the car classes if you want confusion! (Comment offered, you understand, by a bike racer/inspector).
