The compression is measured in mmHg, where mm stands for millimetre and Hg for mercury. It measures how the stockings react at a certain change in pressure. For more information please visit Wikipedia.
There are different standards for compression. Below you can find the defined standard in Europe described.
COMPRESSION CLASS 1
For use with smaller problems like swollen feet, tiredness, and small varicose veins.
COMPRESSION CLASS 2
For use with bigger problems as for instance venous insufficiency and varicose veins. Also for use after operations. Do not use compression class 2 without consulting a doctor or nurse.
COMPRESSION CLASS 3
With major problems you might need compression class 3. In this case the stockings are made to order. Medically qualified personal has to measure the person, so the stockings are a complete fit.
COMPRESSION CLASS 4
Only for use within hospitals.