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I think it's known as load behaviour - this tool may help https://www.nirsoft.net/utils/officeins.html

You may want to read these. https://social.msdn.microsoft.com/Forums/vstudio/en-US/d7dc1409-89a5-44c4-bcee-b3dcc1722a56/outlook-add-in-precedence?forum=outlookdev https://docs.microsoft.com/en-us/visualstudio/vsto/registry-entries-for-vsto-add-ins?redirectedfrom=MSDN&view=vs-2022 https://docs.microsoft.com/en-us/visualstudio/vsto/registry-entries-for-vsto-add-ins?redirectedfrom=MSDN&view=vs-2022#LoadBehavior

Thanks Webster.. I was googling like crazy last night and was coming up with nothing definitive. Closest I got was from that same MSDN Social article that it was synchronous based on alphabetical order of the GUID.

In a past life, I did an Outlook add-in or two and had a couple of books on doing so. It was not trivial.