A Client Access License (CAL) is not a type of software, but rather, a license that grants Users or Devices access to SQL Server software. SQL CALs are a requirement only for the SQL license + CAL model, meaning that if you purchase SQL Server licenses per core (Standard Core / Enterprise Core) you will not need to purchase any additional licenses. The license + CAL model is beneficial for businesses that have a finite amount of users or devices and want to keep the cost of licensing to a minimum. If you cannot count the number of users or devices, it may be best to consider licensing SQL per core instead.