They give clients "permission" to access servers. If a client accesses a server and there's no CAL for that client, you're in violation of your licensing agreement. They're kept on paper basically.

As of December 1, Microsoft is changing the way it prices the "user" option when purchasing client-access licenses (CALs), which will result in higher prices for some customers. With the User CAL, ...

As of August 1, Microsoft will raise the prices of a number of its User client-access licenses (CALs) by 13 percent. The affected CALs are for on-premises, not cloud, products. The prices of Device ...

Microsoft has increased the prices of its on-premises server products and Client Access License (CAL) Suites by up to 20%. Organizations will have to decide whether to stay or go to Cloud products.