I am not sure about the note, as far as I know there is no "AWS Grade 90". I am guessing that the note referr to the tensile strength of the weld metal. Such as E7018, this is a low hydrogen electrode with a minimum 70,000 psi tensile strength. If I am right then the grade 90 may refers to an electrode like E9018 which would have 90,000 psi tensile strength.
The best thing to do would be to ask whoever created the drawing exactly what they meant.

I second Mr. Johnson's comments. My guess would be a welding consumable with a minimum tensile of 90ksi (90,000psi).