NX "Set as Current" is not active in the Reference Sets dialog

While the part is the active display part, open the Reference Sets dialog (menu > Tools > Reference Sets).

The dialog shows the option "Set as Current" as grayed out and not active.

Why is it grayed out, and how to activate it?

The option "Set as Current" is dependent of the context of file.

Below is an example to help illustrate this. Notice the same data (component02) is in the CONTEXT of a higher level assembly here, so it is the Workpart in the assembly file, in this condition, the option "Set as Current" is activated and usable.