Which Standard is "usAISC21.XML" based on?
US.xml, which was originally named US.stf and was likely based on AISC’s 8th Edition (Red Book).
USAISC21.xml, which was originally named USAISC21.stf and based on AISC’s CD database v2.1 that they used to sell most likely from around 2000 or 2001. Likely 9th edition.