Loading Order

Excel Add-ins Dialog Box

For Excel Add-ins the order is defined by the OPEN, OPEN1, OPEN2 registry entries

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 

The add-ins are loaded in numerical order.
OPEN, OPEN1, OPEN2, etc
Excel VBA add-ins are loaded before COM Add-ins

COM Add-ins Dialog Box

For COM Add-ins the order is alphabetical by node

HKEY_CURRENT_USER\Software\Microsoft\Office\Excel\Addins\ 

They are loaded in alphabetical order