The Bottom is the signature which is attached to all the administrative messages sent from the list, such as requests for help, completed subscription, etc …
In other words, it is the signature found in all the replies to the messages sent to:
listname-comand@domainname.xxx.
Whereas the Trailer, is the signature included in all the messages which are sent to the subscribers of the Mailing List.

To edit the Bottom you need to send an empty email to: listname-edit.bottom@domainname.xxx.
To edit the Trailer, however, simply send an empty message to:
listname-edit.trailer@domainname.xxx, making sure to select “Include a trailer at the end of each message” in the Control Panel of the Mailing List