Dave Noble is correct, it's simply the fist character of the booking reference.
If you are on international flights, click this AMADEUS Link and whack that booking reference and your Surname in and hit enter. It should show details of your booking.
If you click the black +'s in little boxes with Orange background; you will get more details, including any allocated seating. Click this and you may even get a seatmap and you can see where you have been put.
It'll also show your ticket number - this is a handy feature so you know you can check whether the agent or airline is holding your money or has actually issued the ticket (including e-ticket). It's also handy if you loose your paper tickets.
The site will also show agent comments - such as flight changes where they want you to contact them and what phone numbers etc. This box only appears if there is a problem.