The Weld Strength Reduction Factor, W, is a temperature dependent value from B31.3/B31.1.
CAESAR II uses this value as:
Wl
A longitudinal reduction factor used in the determination of the pipe's minimum wall thickness.
Wc
A circumferential reduction factor used in the determination of the allowable stress.
The use of W is optional, and is controlled through a configuration setting in the SIF and Stresses section of the Configuration Editor.