Modules

Also known as Extension Modules
Module extension add-ins appear in the Outlook navigation bar, right alongside mail, tasks, and calendars.
link - docs.microsoft.com/en-us/outlook/add-ins/extension-module-outlook-add-ins

<DesktopFormFactor> 
  <ExtensionPoint xsi:type="Module"> 
    <SourceLocation resid= 
    <Label resid= 
    <CommandSurface> 
      <Control> 
        <Action>