Generally, if you believe Wikipedia, then
Цитата:
|
Signs of mandatory fees, together with credit marks (savings, insurance, checking, savings stamps) form a group collection of characters, called "rhubarb." This group is allowed for exposure to the FIP philatelic exhibitions. Special regulations for exhibits class rhubarb introduced PIA to November 25, 1991.
|
http://ru.wikipedia.org/wiki/Фискальная_марка
But I do not understand anything, to be honest.