Gmail, hangouts, play store, chrome, maps, and YouTube all work in multi window, so obviously some of the Google apps were coded for the feature.
I'm not sure why Samsung doesn't make the framework for multi window to include all apps. Developers for custom ROMs can do it, and if you have root there is an app to allow all apps, therefore Samsung should be able to include it as well.
This is probably what I miss most about having root.
I use excel and chrome to pay my bills. Google sheets doesn't work in multi view, so I installed office suite and used an app to make office suite compatible with multi window.
Thank you for correcting my misinformation. Indeed you are correct, upon further checking I see that hangouts, play store, maps, and YouTube do indeed do multi-window. I have never used Gmail or Chrome so I will take your word for it.