We license by the number of User/Checks when you subscribe. Basically, every user you subscribe, comes with one check and one device.
For example, if you subscribe for 5 users, you will get 5 checks and 5 devices automatically. Users refer to the people (phone) that will receive the alerts. Checks refer to the number of configuration (either Ping/Port/URL check or Email Filter rules or Cloud Provider rules) that you can configure. And devices refer to the IP addresses of the server/application that you wish to include in the monitoring.
You can choose from any of them but configure up to the limit that you subscribe for.