When you open an archive in the ArchCommander app, it can open in a new window or as a new tab of the existing window, depending on the user preferences. To set that preference, open the “System Preferences” of macOS and select the “General” applet. There you should see the “Prefer tabs” attribute that shows when tabs are preferred over windows.
If you open multiple ArchCommander windows, you can merge them as tabs of a single window by selecting “Merge All Windows” from the “Windows” menu. You can also turn a tab into a window by using “Move Tab to New Window” from the same menu or by dragging a tab outside the window it is attached to.
