I hope I describing this correctly. In past versions of IMatch was able to assign multiple Categories at one time by checking a list of boxes of all that apply to the group of photos selected. This window was a flyout so I could have it open on another monitor
Can you tell me how to do this now?
See:
The Categories Panel (https://www.photools.com/help/imatch/#panel_cats.htm) and also the The File Categories Panel (https://www.photools.com/help/imatch/#panel_cats.htm#a_current_tab?dl=h-12)
The main help topic for Categories (https://www.photools.com/help/imatch/#cat_basics.htm).
I recommend to use the IMatch help system while you start with IMatch. It should provide quick answers to all your questions.
Don't just try to find your way around, applying what you know from IMatch 3. You will miss many useful features and you may actually work against the system.
Spending a day with the video tutorials and the IMatch help will get you start quickly and smoothly.