User Expression in Part Navigator can not show or hide per selection only in Expression Editor.
Solution
Select in the PNT the user expression and use MB3 to select the Expression in the Editor.
In the Expression Editor the User Expression can set to Show or to hide in the ANT .
Notes and References
NX 1980 What's New in Expressions, Measure, and Mass Props
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: n/a
Product: NX
Application: DESIGN
Version: V1980
Function: EXPRESSION
Product: NX
Application: ASSEMBLIES
Version: V1980
Function: NAVIGATORS
Ref: 002-8587310