Determines whether the attachments in your composition are sent as a part of the mail message or whether those attachments are kept on a local server. If attachments are linked, the recipient(s) of your message will be provided a web link where they can download the attachment(s).
Linking attachments has the advantage of reducing the size of outgoing messages, especially for large attachments. Linking attachments has the disadvantage that the content of the attachments cannot be guaranteed to be available in the future since the attachment data is not located within the e-mail message itself.