High Concurrency clusters are intended for use by multiple users, sharing the cluster resources, and isolating every notebook, etc. For all mentioned languages, separate processes are created and they execute the code that is limited to API exposed by Spark to that language. But Scala code will be executed inside the Spark JVM (per machine) that is shared between all users, so you can get access to everything that is inside JVM.
Typically, when using High Concurrency clusters, the Table ACLs are also enabled (only for SQL or SQL+Python), so you can control who can access which data, enforce row/column level access control, etc.
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