Credits exist as a way of allowing user to create and configure their own trackers on the system.
After purchasing a 'Tracker Credit', it may be assigned to an existing tracker, extending its life by a period of one year; or may be used to create a new tracker for one year.
'Mapping Credits' allow a tracker to use an upgraded mapping service, for example to get aerial photography on the map, or have detailed mapping for Europe etc.
You can purchase as many credits within each category as you wish (within the server restrictions), and can use them over time. They allow simple purchasing and invoicing.