Some lists are set up so that different topics are handled by Mailman. For example, the courses list on Linuxchix.org is a discussion list for courses being run by linuxchix members, and often there are several courses being run at the same time. (eg: Networking for beginners, C programming, LaTeX document mark up.) Each of the courses being run is a separate topic on the list so that people can choose only to receive the course they want to take.
These topics must be configured by the list administrator, but it is the responsibility of each poster to make sure that their post is put with the correct topic. Usually, this means adding a tag of some type to the subject line (eg: [Networking] What type of cables do I need?) or making sure the Keywords: line has the right information. (By default, you can put a Keywords: section in the beginning of the body of your message, but this can be configured by your list administrator.) Note that these tags are case-insensitive.