If the delivery method selected for the documents is 'Email', the Attachments tab will list all documents that will be sent along with the emailed report.
The list also shows the origin of the attachments:
sales offer, purchase order, etcetera: The document is linked to an entity, such as a sales offer or purchase order. The document link settings specify that the document should be included with the report.
Sales offer report, purchase order report, etcetera: The document is linked to the report.
Manual: You can delete or add attachments by selecting a document that is registered on the Documents form. If the document was added on the Send documents processing form, the origin will be 'Manual'.