The Topics function allows to set up what amounts to mini-lists inside your mailing list. This may be useful if a large group or department uses a list for communication. Messages sent on such a list might include information on budgeting, meetings, projects, attendance, and more. For such a list, you could set up topics such as 'budget'. Mailman would then scan the body of the message for the word 'budget' and if found, would tag the message as a 'budget' topic message. Users could then decide to not receive message flagged as budget messages.
The topics_enabled option controls whether Mailman should scan for topics in the body of incoming messages.
The topics_bodylines_limit tells Mailman how many lines of the body is should scan for a match.
The topics option allows you to add topics for Mailman to scan for in the bodies of messages sent to the list. You can have multiple topics. Mailman will tag a message with the first match it finds. For instance, if you have topics for both 'meeting' and 'budget' the message 'We will have a budget meeting' will be tagged by whichever is first in the list of topics. The topics option uses regular expressions to find it's matches. While many departments choose to have separate mailing lists for sub topics such as this, this function may be useful if you'd prefer to have all department or group message be sent to one list so they are all archived together.