In older versions of NX (e.g. NX12) the "Create interpart link" button was in the selection menu and selectable as a function directly.
In NX 1946 / NX 1953 the function is always there but "greyed out":
Get the button back go to the Top Border Bar go to -Selection Group - set the hook by "Create Interpart Link ".
now you will find the function under the selection menu.
Example:
To use the function, you must be in the "Entire Assembly" and then select any other Assembly / Wave function.
Only then the button becomes "active" and a link can be created.
After using the function, the button is grayed out again!
Additional Information:
Now we have added a new functionality called "Automatic work part change(AWC)". You can find this toggle just before Interpart toggle button on selection tool bar. When the AWC is enabled then there will be no inter link allowed and hence it is greyed out. If you disable the AWC toggle before the command then you can see that you are able to create the inter part link.
Notes