As JS notes, each manufacturer has their own requirements for the weld necessary to complete the joint between the integrally reinforced branch fitting and the run pipe.
The integrally reinforced branch fitting is designed to replace the volume of metal removed from the run pipe by the hole for the branch fitting. One alternative to the IRBF is to reinforce the opening with rolled plate. The alternative is typically more time consuming and expensive than the use of the integrally reinforced branch fitting. Since each manufacturer has a different design and since each has been tested by the manufacturer to demonstrate it performs as intended, each has different welding requirements. The IRBF only functions properly if the weld is made in accordance with the manufacturer's requirements.
The designer should include the welding requirements on the drawing, but since few designers have a working knowledge of welding, they fail to deliver the goods in that respect. It is my opinion that the designer that specifies the use of a particular brand of IRBF should also be responsible for finding out what the welding requirements are. The contractor has a choice of including the welding requirements in the WPS or in the drawing package. Ultimately, someone has to take responsibility to provide the information the welder needs to weld the fitting properly. It is not the welder's responsibility to determine how much weld is necessary.
It is rare that I find the IRBF welded properly. I typically hear comments to the effect that the welder doesn't feel more weld is necessary because the system only carries XX pressure or that additional weld causes the run pipe to bow. Unfortunately, the welder is usually the least qualified individual to determine how much weld is necessary, but the designer is apparently a lazy sod or even less qualified than the welder based on the absence of the drawing information.
I have always advocated using the WPS to provide the direction needed by the welder. Whether it is required by the applicable code is immaterial. It is information needed by the welder. Since the contractor is often tasked with purchasing the pipe and fittings, it is appropriate for the contractor's designer (assuming someone in the contractor's employ fills that function) to provide the information to the welder.
Best regards - Al