What is a Microsoft SQL Server CAL and what is it required for?
An SQL Server CAL is required to licence access to an SQL Server from Microsoft. It is an essential part of Microsoft's licence policy and is required for the legal operation and use of the server software. Companies that use Microsoft SQL Server in the Standard Edition must acquire a licence for the operation of the server itself as well as a sufficient number of access licences (CALs) and provide proof of this during an audit.
Device CAL or User CAL - differences and similarities
Microsoft's licensing policy provides for two different types of client access licences: User and Device CALs. A Microsoft SQL Server Device CAL licences a device that can be used by multiple users to access and interact with the SQL Server. In contrast, a Microsoft SQL Server User CAL licences a defined user who can access the server from different devices and use its functions.
You are free to choose whether you cover your licence requirements with User and/or Device CALs; it is also possible to combine both CALs. However, this only really makes sense in a few cases. For cost reasons alone, it is almost always better to opt for one type of licensing.