info about casuser lib

docs/libraries.md

@@ -26,6 +26,9 @@ Library definitions should be added in the `autoexec.sas` of the designated Comp
 
 If the above is not feasible, it is possible to insert code in the `[DC Drive Path]/services/settings.sas` file however - this will have a performance impact due to the additional API calls.
 
+!!! note
+    The CASUSER library must be assigned, as it is used to store temporary in-memory tables during CAS data updates.
+
 ## SAS 9 EBI Libraries
 
 In most cases, libname statements are NOT required so long as they are accessible in metadata.