When you route pipes, the system automatically creates external references if necessary.

An external reference (or an external dependency) is a relationship between an object (part, subassembly) and some information from another object that is not inherently available to the referencing objects all of the time.

Creo Parametric always creates an external reference in the context of an assembly, where some information is referenced that is external to the model you are changing.

Whenever you retrieve the subassembly, the system automatically retrieves all of the components that belong to it. Therefore, information needed to place features and components is available.

However, if the subassembly existed among some other components in a higher level assembly, and some of those other components that are members of the higher level assembly—but not the subassembly where the feature is being added—were referenced, the subassembly would have an external reference.

The subassembly would never know exactly where the feature should be placed unless the subassembly was currently in session in the context of the top-level assembly.

For example, if you create a Pipe To Point feature using a datum point in a part that is in a subassembly other than the one with the piping feature, the pipe routing feature now has an external reference.

This feature belongs to the assembly where it was created and every time you regenerate the assembly, Creo Parametric looks for this feature’s references in the part in the other subassembly.

All of the information from the assembly is available during this regeneration. However, this dependency of the pipe routing is accurately and completely updated when it is in the context of the assembly containing both.