This blog post outlines how to create your own CAS functions using the CAS Language. It also includes a partial list of both CASL built-in and common functions for reference.
Customize your CASL code with built-in and user-defined functions
How to use CASL to develop and work with user-defined CAS actions
In addition to the many CAS actions supplied by SAS, as of SAS® Viya™ 3.4, you can create your own actions using CASL. Learn how in this post.