Hi,
Did you check the official document?
https://docs.uipath.com/activities/other/latest/user-guide/n-application-card
Single Window mode is similar (same?) with previous AttachWindow / Borwser activity.
It will search element in the specific window.
Application instance will search element through all the windows in the application instance (process).
As application instance mode is less number of activities than Single window mode in case of switching window, in most case it’s good to choose application instance, i think.
Regards,