Visual Studio Tools for Office CommandBarButton FaceId Property
图标右侧的数字是要使用的相应FaceId:Icons 0000 - 2999
Outlook FaceId 3000 – 3999
Reference
http://www.kebabshopblues.co.uk/2007/01/04/visual-studio-2005-tools-for-office-commandbarbutton-faceid-property/
http://www.kebabshopblues.co.uk/2007/01/24/outlook-faceid-3000-3999/
https://docs.microsoft.com/en-us/previous-versions/office/developer/office-2003/aa159658(v=office.11)