The class names are defined in a DEF file in one the sub-folders of %UGII_BASE_DIR%\MACH\resource\library\tools. The sub-folder depends on the type of tool library you are using.
To change that create a copy of the entire resource folder on a server share and point NX to that folder with your changes. For further information about creating a customization, see the NX_custom package mentioned in the NX customization forum.
It is generally bad habit to change things in the installation folder of NX, since software updates will overwrite your changes without notice. In addition, you would have to apply those changes on each computer manually.
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First, be sure you read the help section on CAM Library Terminology. These are classes.
There is a lot in the help an adding tool classes and subclasses in the ascii library. I'm not sure about changing the name of existing classes.
This customization requires editing the tcl and def files that define the library organization and access. Remember that if you customize the classes, you will need to migrate your customization with each new release.
Why do you want to change them?