An email message attachment is a file sent with an outgoing email message. Standard activities and functions allow outgoing email messages to include attachments. See How to send email with attachments.
A work object attachment identifies a named small file, screen shot image, correspondence, URL or text note that a user or an activity has associated with a work object.
On the standard harness Perform and Review forms, a paper clip button provides access to add, edit, delete, or review work object attachments. Correspondence attachments are permanent parts of the history of the work object and cannot be deleted or updated.
A rule attachment is a file associated with a rule, saved as a separate (binary file rule).