can any one provide me with the precise definition of "the maximum thickness of shell or head plate" stated in UCS 56-5.

I would make the assumption that the intent of UCS-56(d)(1) through (5) is intended to reference the "governing" metal thickness as defined in UW-40(f)(1) through (6).
My opinion is based on the following interpretation which asks a question regarding the term "maximum metal thickness" with the reply stating "governing metal thickness"

Interpretation: VIII-1-89-186
Subject: Section VIII, Division 1 (1989 Edition), Nominal Thickness Requirements to Comply with PWHT Procedures
Date Issued: April 3, 1990
File: BC90-305
Question: In using the attachment detail of Fig. UW-13.2 sketch (d) in Section VIII, Division 1, can the weld joint thickness of a shell to tubesheet attachment be considered the maximum metal thickness in determining the rate of heating and cooling for postweld heat treatment?
Reply: No. See UW-40(f)(5)(c).

FYI this is the only interpretation I have found discussing heating and cooling rates for ASME VIII-1
As an added note, ASME Section I specifies the heating and cooling rates within the Tables of PW-39. This would imply to me that ASME Section VIII-1 takes a more conservative approach by not including the heating and cooling rates with the Tables of UCS-56 , but rather puts the requirements in separate paragraphs. I can understand your confusion. I would recommend having a concensus vote as to the intent of this paragraph if you have differing opinions.

My answer was confirmed here at the following link: http://www.eng-tips.com/viewthread.cfm? ... 689&page=1

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