I am looking for advanced documentation or FAQ on co-authoring with Word on Office 365. For example, what happens when tracking changes is enabled. Or common issues and workarounds on low-bandwidth connections. Any pointers would be appreciated.
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There's lots of resources online that you can have a read of (also has links that lead to other useful articles):
Troubleshoot co-authoring in Office
Document collaboration and co-authoring
Overview of co-authoring in SharePoint Server
If you web search the keywords, you will find a multitude of other articles and it also indexes community topics like this so you'll be tapping into the knowledge that has previously been shared.
Hope that helps and best wishes!