In game there could be KE (kinetic energy, es. APDS), CE (chemical energy, es. HEAT) and tandem penetrators. Each interact differently with armor, which also have 3 types of "values".
Let's imagine a hypothetical tank with 200mm cast armor and Kontakt-1 ERA. KE value (vs. KE penetrators) is 200mm, however CE value (vs. CE penetrators) will be significantly higher, let's say 500mm, and will probably stop many regular small caliber HEAT rounds (although ERA might not be the best example as it works by degrading penetration in % in real life) . However, TANDEM value protection will be the same 200mm, as it will neutralize 1st Generation ERA. However, tank can also be hypothetically equipped with some different type of passive CE armor that might provide protection against tandem as well.