What is a Fitting?
A fitting is a not a single entity in Spec-Driven Piping. It is actually the group of fitting instances that have been deemed similar enough to group together. Rationale for grouping instances into a fitting include matching function, and geometric similarity, Let's look at an example from the RsysConfig FITTINGS page.
Tee Straight Bw Steel is one of the fittings in the example Master Catalog. The Fitting information page shows what metadata all of its instances have in common: Name, Selection Name, Code, Category, Material (AA-Steel), and Unit System.
The Tee Straight Bw Steel instance page shows all the instances for that fitting.
The instances can vary in schedule, model name, and size and end type for each end port. Each instance must have a unique combination of schedule and end port sizes. For example, if there are two instances with the following:
Spec-Driven Piping will ignore the second fitting instance,although it references a different unique model and has different end types. The second instance is not considered unique because its combination of schedule and end port sizes is not unique.
Which instances are included in a fitting is determined by the user. Often fitting instances align with model families. This example, Tee Straight Bw Steel, uses many of the instances from the family table for TEE_STRAIGHT_BW.prt. However, the copper instances are excluded. Instead, because of their material difference they be matched with instances for a different fitting with copper as the Spec-Driven Piping material. If the difference was not material, but instead was some code for surface finish, and the size and schedule combinations matched those of other instances, those parts need to be grouped into their own fitting in RsysConfig.
There is no requirement or recommendation that Creo family tables be used as the basis for fittings. A fitting certainly can and in many cases do include geometrically different models that are otherwise unrelated in Creo, as long as the schedule+sizes combinations are unique, For example, an organization might have one valve supplier for size 10 - 50, another for sizes 50-200, and a third for any above size 200. All of those different parts can be instances of the same fitting.