From: Charles Wilt
For that matter, every single RDBMS I'm familiar with only allows a single level
I gather that you mean the "catalog" or "database" parameter referenced on RDBMS "connection" strings. At any rate, I like the analogy. They don't offer an option to define or connect to a database within a database within a database. Likewise, IBM i doesn't offer an option to define or connect to a library within a library within a library (with the exception of QSYS).
Referencing files and other objects within IBM i libraries is essentially similar to referencing RDBMS objects within a "catalog" or "database".