This setting informs the system which of your archive maps is preferred for an archive type which has more than one template assigned.

Movable Type allows you to have multiple archive maps for each type of archiving, Category, Date-based and Individual. However, when you link to an archive type (e.g. using <MTArchiveList archive_type="Individual">) which has multiple archive maps, the system must know which of the archive maps is preferred in forming links. The radio button next to the archive maps is used to set this preference.

It's important to note that this choice is only available if you have more than one archive map assigned for a particular archive type. Otherwise, the single existing template map is your default preference.