Send Permissions allow admins to designate the Direct addresses or fax numbers users are allowed to send from.
Users will not have a 'From' option to send messages from until their admin grants them Send Permissions. This can be done as you're creating the user account, or later as you're editing the user account or the Direct address/Fax number.
Edit User's Send Permissions
Navigate to Settings > Users.
Click the Edit button (pencil icon) to the right of their email address.
The Global Send Permission(s) field will display any fax numbers and/or direct addresses configured for all active users. (Please note: Global Send Permissions cannot be edited from the user screen.)
Use Additional Send Permission(s) to add desired fax numbers and/or direct addresses. (Please note: Globally assigned numbers and addresses will not be selectable in this field).
Save Changes.
Edit Address or Fax Number Send Permissions
Navigate to Settings > Addresses (for Direct addresses) or Settings > Fax Numbers (for fax numbers).
Click the Edit button (pencil icon) to the right of the address or fax number.
Select a preferred option for the Assign this rule to: radio button.
Select Global Rule (applies to all users) to grant all active users permission to receive notifications and view messages. Once set for a rule, all newly added users will automatically be included in the rule permissions.
Select Specify a list of users to grant specific users permission to receive notifications and view messages.
Save Changes.