In which part of AISC is this written? I can’t seem to find it in Section J4.3, nor in its corresponding commentary.
[This is taken from AISC Design Examples v15]
In the example you’re looking at, the stem extension plate is grade A36. The mechanical properties of which are standardized by ASTM - minimum first yield at 36 ksi and minimum ultimate at 58 ksi. You won’t find that in AISC 360 (Specification for Structural Steel Buildings). Now, if you have the steel construction manual (which contains the AISC 360), there are tables that list mechanical properties for various steel grades (in my 13th edition of the SCM, table 2-4 is Applicable ASTM Specifications for Plates and Bars).
The AISC 360 is written is such a way as to be useful no matter what steel grades you’re using (within any limitations noted). For instance, the stem extension plate could have been ASTM A572, which would have Fy = 50 ksi and Fu = 65 ksi.
Above is a snippet.