Here’s a short description on the differences between these similar options.
The option Multi-window support sets a flag to indicate the app is split-window capable. The Android system will then allow the app to be used in split window configuration. Normally this is limited to two apps visible at the same time.
The option Free-form window enables launching the app as a floating window but the implementation of these floating windows comes from the Android system and requires Android 7.0 or higher.
The option Floating app uses a custom floating window implementation that also works prior to Android 7.0 and may, in some cases, work better than free-form windows.
If you want to keep apps running and active in the background (e.g. prevent videos from being paused as the focus moves to another app), you should also consider enabling the Multi-window no pause option. This can be used in conjunction with any of the above options.