Specifies the level of API access for your Office Add-in; you should request permissions based on the principle of least privilege.
Add-in type: Content, Task pane, Mail
For content and task pane add-ins:
<Permissions> [Restricted | ReadDocument | ReadAllDocument | WriteDocument | ReadWriteDocument]</Permissions>
For mail add-ins
<Permissions>[Restricted | ReadItem | ReadWriteItem | ReadWriteMailbox]</Permissions>
For more detail, see Requesting permissions for API use in content and task pane add-ins and Understanding Outlook add-in permissions.