B31.3 Paragraph 331.1.3 Definition of Thicknesses Governing PWHT

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Dear Expert ,

kindly help me to explain what mean control thickness in below paragraph 

331.1.3 Definition of Thicknesses Governing PWHT
(a) The term control thickness as used in Tables
331.1.1-1 and 331.1.3-1 is the lesser of
(1) the thickness of the weld
(2) the thickness of the materials being joined at the
weld or the thickness of the pressure-containing material
if the weld is attaching a nonpressure-containing material
to a pressure-containing material.
(b) Thickness of the weld, which is a factor in determining
the control thickness, is defined as follows:
(1) groove welds (girth and longitudinal) — the
thicker of the two abutting ends after weld preparation,
including I.D. machining
(2) fillet welds — the throat thickness of the weld
(3) partial penetration welds — the depth of the
weld groove
(4) material repair welds — the depth of the cavity to
be repaired
(5) branch welds — the dimension existing in the
plane intersecting the longitudinal axes, calculated as indicated
for each detail using the thickness through the weld
for the details shown in Figures 328.5.4-2 and 328.5.4-4.