All bevels have a set of reference part parameters. Butt welds have an additional set called non-reference part parameters. For butt welds, a single bevel is applied to both parts. It is necessary to apply a set of values to each part because they are sent to manufacturing separately.
You set the reference part in the physical connection parameter rules code. This can be decided by a user-defined convention. As an example, the delivered rules decide that the thinner of the two parts is always the Reference Part. If the parts have equal thickness, then the first part in the connection is labeled as the reference part.
Though the rules can decide which part is the reference part, the rules must also report the result to manufacturing, by setting the ReferencePart parameter. Structural manufacturing uses the order of the two parts in the connection.
If ReferencePart = 1, manufacturing applies the reference bevel parameters to the first part in the physical connection
If ReferencePart = 2, manufacturing applied the reference bevel parameters to the second part in the physical connection
The first part in the physical connection has a thickness of 20 mm. The second part in the physical connection has a thickness of 10 mm. The second part gets the Reference parameters and the ReferencePart is set to 2. The first part gets the Non-Reference parameters.