Regarding to the mandatory check out, my understanding was that even when this option is turned off,
at least in case of Excel files, you cannot edit them simultaneously as the default opening client is the local client
and not the online app. I checked it and even with check out off, the file is blocked for editing if someone else is already editing it.
But on the contrary in the versioning settings I saw a section :
Require Check Out (for library versioning only) à Select this option if you want to protect documents in this library from being modified by multiple users at the same time. Users will have to check out documents before editing them, and then check them back in when done.
Can anybody please explain this ambiguity.