Today I saw something on a customer's machine I never did before as I didn't know this options exists - the grouping of windows in the Notes client.
The goal of this setting is to reduce clutter and the amount of open windows (tabs) in the Notes workspace. If enabled, you now have grouped windows and your are able to close them all at once.
The caveat: the "x" is removed to close a single window even if there is only one of a certain application.
I will not use this feature as it has too many downsides to me but if you feel it, then just try it!
Have a nice weekend!