When you open a Task Assignment request letter, the Assignment Reply buttons are displayed:
If you click the Accept or Tentative button, a positive reply is sent back to the Task organizer, and the Task is copied into your Main Tasks folder (Mailbox).
If you click the Decline button, a negative reply is sent back to the Tasks organizer, and the Task is not stored in your Main Tasks folder.
You may want to enter a comment into the panel text field.
If you click any of the Accept/Tentative/Decline buttons, the original request letter is deleted.