I found the paragraph that spells out revision sequence type.
ASME Y14.35M-1997
Section 5 Identfying Revisions on Drawings
5.1 Revision Letters
Upper case letters shall be used in sequence beginning with A and omitting letters "I"O", "Q", "X", and "Z". When the single letters have been exhausted, the revisions following "Y" shall be "AA", "AB" through"AY". Should "AA" to "AY" be exhausted, the next sequence shall be "BA", "BB", etc. Revision letters shall not exceed two characters. Initial issue of a drawing does not constitute need for a a revision letter and may be indicated by the use of a - (dash).
there is more, but this covers the basic principles.
"Wildfires are dangerous, hard to control, and economically catastrophic."
"Fixed in the next release" should replace "Product First" as the PTC slogan.
Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand