Plants are marked in a slightly different way, in different places, but the most common are P markings and C markings. Some mark the volume of the pots in liters.
So what do these mean?
P9, P11, etc., means a 9cm pot, or an 11cm pot. P stands for POT, i.e. pot in Finnish. P9 is about 0.5l.
C markings, on the other hand, are often how many liters the volume of the pot is, i.e. C3 is about three liters. C stands for CONTAINER, i.e. container, pot or similar in Finnish.
Some also sometimes state the size of the pot in millimeters, such as 180, 230, etc. I think this is a bit of a tricky way to understand the size of the pot. That 170-230mm can be about C3, i.e. a 3-liter pot, but then there are different heights, squares, or a centimeter or two for the holes in the bottom.
The P markings are usually pots for 1-2 year old seedlings and the C marking pots are moved to when the plant starts to be a good size.
The size of the plant is usually stated as e.g. 10-30cm and this is the height of the plant without the pot. Plants can vary a lot because not all individuals are equal.
Faster-growing individuals usually get to a bigger pot earlier and the mark itself, e.g. P9-C3 24, which means which pot was changed to which size and in which year. These individuals are therefore faster growing and usually stronger than other seedlings growing in the same lot!
In this picture, the Pinus Strobus "Pendula" grown in the P9 pot gets into the C3 pot. This individual was an individual that grew more diligently than its fellows.