See UG-37(a), definition of "F" factor. Then see definition of "integral reinforcement" given in UW-16(c)(1).
Generally, a separate loose "reinforcing pad plate" does not meet the requirements of "integral reinforcement". Thus you cannot use the F factor to gain benefit of reduced reinforcing area through cross-sections not taken through a longitudinal "cut". But may not be a big deal.
Generally, you would use integral forgings, a special forging that has the nozzle neck (and possibly the flange too) integrally forged with a heavier barrel to provide additional reinforcing area when separate plates are not desired. This may be for fabrication cost savings or other reasons (high temperature process, etc).
My understanding is that intergral or self-reinforced nozzles are most often used in cyclic services. The major reason is less discontinuties and fillet welds etc, which always introduce the higher SCF and peak stress.
In addition to the forgings, insert-plate type nozzles are used very often also, since only full pen welds exist.
