_Customization vs (Customization) in Menu Maintenance

Struggling to get the PartLookup form opening up with a customization…It’s a handheld form called from the Location button on the Container Receipt line. The only difference between other menu items I see. is the fact that the customization field has (Customization) in the customization field in menu maintenance vs one that works the customization id with a _Customization appended to it:

This one works
image

This one doesn’t and opens the standard form:
image

I had to use process calling maintenance and create a new menu to assign the customization to it.

I am very interested to know why there is a difference.

Any help appreciated.