NFPA 68 Question

Will someone shed some light on a section of NFPA 68 - 2007. In Section 6.3.1.3.2 (1) there is a varible F sub u deifined as

"ratio of ultimate stress of the enclosure to the allowable stress of the enclosure per the ASME Pressure Vesssel Code"

and in section 6.3.1.3.2 (2) there is a varible F sub y defined as

"ratio of the yield stress of the enclosure to the allowable stress of the materials of construction of the enclosure per the ASME Pressure Vesssel Code.

I have looked in ASME BPVC Section VIII Div 1 and I can't find either F sub y or F sub u. Can some one point me to the location of these variables in the code?

Thank you.