Plant 3D is designed to work
intelligently
with Xref's. Pipes and Piping Components will connect to pipes and piping components, and even equipment nozzles, in Xref models. For example, when using the Route Pipe command, by clicking on the equipment nozzle in the Xref, the connection will be made and the correct components will be placed as shown here: 
Note that the pump is in the equipment Xref and that since we were routing an 8" line and connecting to a 10" nozzle, a 10" mating flange with a 10x8 reducer was placed in the piping model. Also note the connection was also placed which will give us the correct bolts and gasket.
Modeling Using Xrefs
Xref's make it very easy to work in large models. Here the piping designer is concentrating on piping around a pump: 
By unloading the structure and turning off some of the piping layers, a better view is obtained making piping easier: 