Letters on Lorries

[Shortyshort]

Hi Guys & Gals,

I'm new to this forum thing so if I got it wrong I'll apologise now.
I'm hoping some of you good natured people may be able to help,I have recently been abroad and whilst there I saw lots of lorries that had a big square with a letter inside (either d or a or r or b )this was positioned either on the side or the back of the trailer and came in green,blue or yellow , would somebody out there be able to shine light on this a tell me what they mean.Thanks in advance for the help from the Newby