If you are implementing add-in commands in your Outlook web add-in to declare custom Ribbon buttons you need to make sure it doesn’t contain any deprecated elements from the Preview when it first appeared nearly a year ago. If you don’t and you try to submit your app to the Office Store your manifest will be rejected due to a schema violation if you are using the ToolTip element. This element used to be valid in the Group and Control extension points but have been removed.
I believe the documentation and samples may have still been referencing this element up until just lately, but I haven’t got confirmation from Microsoft yet on exactly when it was removed. You can see an old example on GitHub that references it, but the repo’s README is quite clear that it is out of date. I strongly suspect this change was quite recent because the app submission page allowed my manifest when I uploaded it on April 17. When I tried to submit a revised manifest today (I stupidly forgot the SupportURL and HighResolutionIcon elements; check the checklist!!!!) it failed and pointed directly at the ToolTip element. So it’s now useless!
Note also that the Office App Compatibility Kit still validates manifests with this element!!! It hasn’t been updated since it was released on April 27, 2015. Which is the same day as today. Hmmm… coincidence? Maybe the add-in commands preview had a full year to live until something kicked in internally at MS. Anyway, I have pointed this out to Microsoft so hopefully the tool will get updated soon.