Sorry to bump this thread, but after having a brainwave and searching with google translate and asking real truckers what they meant on a service station at the M1(A), I have come up with Finalized evidence.
I found out that multiple countries have different laws. Here is a list.
IN FRANCE:
60,80, and 90 on white discs must be displayed, as pierrot14 said.
IN ITALY:
An 70 and 80 must be displayed on a white disc with a red border to show speeds for country roads and Motorways.
IN BELGIUM:
A 60(km) sticker must be displayed to show that is the speed for country roads.
IN CZECHIA:
A sticker must be displayed showing a vehicles limited speed, i.e 90(km) for a normal EU lorry.
IN EASTERN EUROPE:
Bored police officers will fine for simply ‘not showing speed limits’ so it is recommended to show all of the limits in your country (e.g 50, 70, 100 for Bulgaria)
IN THE EU:(Technically)
Vehicles over 5 tonnes gross weight should show a sticker showing their maximum design speed.
So, if you wanted to be really safe, you would have to show 50,60,70,80,90, and 100!! 
Of course, this is rarely spotted. Companies will usually slap on a combination of stickers for safety, and trucks going international can sometimes be seen sporting nothing on the backside. after all, even though only the prattiest of police would pull you over because of a bloody sticker 
and that, after a full 1 month of research, is a complete guide to why you see stickers on the back of trucks! 
Im thinking about making a website covering this, as well as other stickers and plates and signs that you might see at European borders.
also pierrot14, what an amazing coincidence 