Chris
These steels, which are called High Performance Steels (HPS), have nominal yield strengths that range from 485 MPa to 900 MPa, yet exhibit excellent ductility, toughness, and corrosion resistance. HPS can be welded with greater ease than many steels developed in the past. Therefore, HPS is an excellent candidate for bridge construction, and in fact is being used to develop many innovative bridge designs HPS can produce cost
savings in bridge construction because the number and size of girders can be reduced due to the increased strength. The toughness and corrosion resistance of HPS makes it particularly attractive for bridges.
Hope this helps more than hurts.
Good Luck
Marshall

Typically HPS is Gr. 70 or above, but someone wanted HPS 50W (in my mind, just an excuse to make steel more expensive, but no one asked me), so there it is.

It is like 50W but tighter chemical controls and higher toughness.

All that stuff in 5.4.2 is all about making sure your PQR plate is not a best-case scenario (if anything, you want it to be a worst-case scenario), so don't order HPS 50W for your PQR plate.  If you take a look at the table in 5.4.1, if you run your PQR with regular 50W, you can qualify any Gr. 50 (including HPS 50W) or Gr. 36.  If you run it with HPS 50W, it's only good for HPS 50W and won't qualify downward.

Hg