The PL/SQL compilation and run-time system is an engine that compiles and executes PL/SQL blocks and subprograms. The engine can be installed in an Oracle server or in an application development tool such as Oracle Forms.
In either environment, the PL/SQL engine accepts as input any valid PL/SQL block or subprogram. Figure shows the PL/SQL engine processing an anonymous block. The PL/SQL engine executes procedural statements but sends SQL statements to the SQL engine in the Oracle database.
It is a separator,which splits the SQL and PL/SQL statements.PL/SQL block comprises of SQL &PL commands are submitted as one request.PL/SQL engine split the block and sends appropriated commands to the corresponding units.SQL evaluator will handle SQL statements(available on the server side).PL Evaulator to handle PL commands.Engine can reside either in client side or in the server side.
Where PL/SQL stands?
The client can interact with the system through front end application program.When there is a data oriented operation it invokes the PL/SQL program in single step.In PL/SQL end it approaches PL/SQL Engine which splits PL and SQL part separately and send the SQL request to the SQL.
Now SQL sends the data request through query to database.Data if available will be send back to SQL in turn to PL/SQL.Now PL/SQL will send the return values back to the client application