With the latest release of NX you will find many new enhancments into our core design area. The automatic work part change is an all new functionality to NX. Modeling within a single part and in the assembly context becomes significantly easier through a number of efficiency improvements such as the automatic work part change. The automatic work part removes the need to explicitly set the work part. This will reduce RMB interactions, while speeding the design process in assembly workflows.
The work part is automatically set based on selected geometry
See here for a demo on this functionality:
NX Help Documentation - Allow Automatic Work Part Change