Trade types (T, B, X and M) - What do they mean?

I've been trading/investing in shares for the last year or so and the one area that my knowledge seems to be lacking is trade types. I just dont quite understand what is happening when a '(B) broker to broker' trade gets executed or a '(X) Agency Cross or Riskless Principal at Same Price' gets executed.

I quite often see trades like:

14:53:01 473p 250,000 £1,182,500 X

but i just haven't a clue what is happening.

I would love to be able to know exactly what is happening when a trade such as a B or an X gets executed.

Anyone?

...

Thanks for that, the document explains a little more than I'v found anywhere else.

What I really want to know is are these sorts of trades good indicators or bad. So say a "broker to broker" trade, does this indicate that a broker wants some stock for a big buyer (a good sign)? mean nothing at all?

Same for all the other trade types. I just dont understand if these type of trades are good or bad indicators for the future price of share.

I doubt you can attach much meaning to them consistently, otherwise you'd be getting a free lunch. If there were a consistent strong correlation between a big B trade and subsequent tradeable price action, it'd be hit by arbitrageurs and others faster than you could respond. Just guessing though - others will know much more than I.