Answer:

The Joint Efficiency is determined in AutoPIPE Vessel following the rules of ASME Sec. VIII, Div. 1, UW 11, 12 AND 13.

The joint efficiency is a function of weld category, weld type and extent of radiography in ASME Sec. VIII div. 1.

Under Vessel Properties Window > Design Conditions tab the RT type is specified (Full/Spot/None) and also the joint type (Type 1/ Type 2).

So for a longitudinal seam weld of shell which is a category 'A' joint and with Full RT and Type 1 weld, the joint efficiency as per Table UW-12 should be 1, which is reported and used for calculation in the report:

Now if the degree of radiography is changed to "Spot" and the weld type changed to "Type 2", in the Design Conditions and recalculated, the calculations correctly update with  joint efficiency = 0.8:

AutoPIPE Vessel also allows user defined joint efficiency input for longitudinal and circumferential welds: