Custom allows you at add text or descriptions to the Email.
snip from Admin guide:
The Custom Email Alert sends a fully configured email message to a list of
recipients. The message body, subject line and email headers can all be completely
customized in accordance with the RFC 822 specification, which defines a
standard format for electronic messages consisting of a set of header fields and
an optional body. Messages can be made complex and rich with formally-structured components of information or be kept small and simple, with a minimum of such information.