The Pay Per Use feature captures license usage in a pay-per-use database which can then be sent to the vendor upon request. The database is authenticated, and its contents can be validated by the vendor.
For example, if you have an unrestricted use license that is paid for based on monthly usage, your vendor can request that you write the usage information to a pay-per-use database each month, and send the database file to them for billing purposes.
Usage databases use SQLite, a free open source database. You can find out more about SQLite at http://www.sqlite.org.