There is no clear definition of high strength steel and there is no European standard either. So it is up to each steelmaker to define the various strength groups. Steels can be classified into the following strength groups, with the yield strength as the criterion:

Mild Steel (MS)
High Strength (HS) steel
Extra High Strength (EHS) steel
Ultra High Strength (UHS) steel

We use the grouping in accordance with the SSAB Business Steel School for classifying sheet steel.
What distinguishes the steels are the mechanical properties, and principally the yield strength (Re).