This is all I could find,
From an article
Use Caution When Specifying "Seal Welds"Welding Innovation Vol. XVI, No. 2, 1999
Practical Ideas for the Design Professional by Duane K. Miller, Sc.D., P.E.
"The American Galvanizers Association (AGA)
defines three classes for welded assemblies that will be
galvanized. Class 1 Joints are held together by a full seal
weld. Class 2 Joints are held together by seal welds, but
the overlapped area is large enough to require venting, i.e.,
provision of an escape hole for the release of expanding
trapped gases. Class 3 Joint details do not contain seal
welds."
Here is the link to the full article.
http://www.jflf.org/pdfs/papers/design_file6.pdfAnd also
ASME B31.8 SYSTEM CLASS CRITERIA Note page 9
http://engstandards.lanl.gov/engrman/13weld/pdf/2-01_Att.3-R0.pdf