I'm not 100% positive but here is what I think it means.

• The letters are the Coach Letters.
• Letters in inverted type are first class coaches, those in normal type are standard class coaches.
• The cup and bycycle indicate the location of the onboard shop and the cycle storage (Coaches C and A)
• The end that is chamfered to look like a driving cab is the front of the train.
• The amount of fill on the coach pictures indicates roughly what proportion of seats in that coach are reserved. If you don't have a reservation you probablly want to aim for a coach with few reservations to maximise your chance of getting a seat.

I'm not 100% positive but here is what I think it means.

• The letters are the Coach Letters.
• Letters in inverted type are first class coaches, those in normal type are standard class coaches.
• The cup and bycycle indicate the location of the onboard shop and the cycle storage (Coaches C and A)
• The end that is chamfered to look like a driving cab is the front of the train (furthest from the concourse at Euston).
• The amount of fill on the coach pictures indicates roughly what proportion of seats in that coach are reserved. If you don't have a reservation you probably want to aim for a coach with few reservations to maximise your chance of getting a seat.

This is all based on personal experience. I don't know of any official source.