Oracle Database – Multiple Instances or One Instance with Multiple Schemas

As your own question answers: "Oracle Applications is an [ERP]." An "Oracle Apps DBA" will be someone who is experienced in working with the particular schema and tooling around this ERP. There are hundreds (if not thousands) of packages, tables, stored procedures, and interfaces that are specifically built for the ERP. It is not a trivial database to admin. You can get a brief glimpse into some of the things in their ERP by looking around - for example, this blog - or by logging into a Metalink account as someone who is a customer of their ERP solution, where you can get ERDs and other documentation.

You could encapsulate the logic into the packages themselves:

SQL> CREATE OR REPLACE PACKAGE pkg IS
  2     g_context VARCHAR2(30);
  3     FUNCTION  get_ctx(p_variable VARCHAR2) RETURN VARCHAR2;
  4     -- set_ctx may be private of course
  5     PROCEDURE set_ctx(p_variable VARCHAR2, p_value VARCHAR2);
  6  END;
  7  /

Package created

SQL> CREATE OR REPLACE PACKAGE BODY pkg IS
  2  
  3     FUNCTION  get_ctx(p_variable VARCHAR2) RETURN VARCHAR2 IS
  4     BEGIN
  5        RETURN sys_context(g_context, p_variable);
  6     END get_ctx;
  7  
  8     PROCEDURE set_ctx(p_variable VARCHAR2, p_value VARCHAR2) IS
  9     BEGIN
 10        dbms_session.set_context(g_context, p_variable, p_value);
 11     END set_ctx;
 12  
 13  -- set ctx dynamically
 14  BEGIN
 15     g_context := 'MYCTX_'||sys_context('userenv', 'current_schema');
 16  END;
 17  /

Package body created

Let's create the context and call it :

SQL> BEGIN
  2     EXECUTE IMMEDIATE 'CREATE CONTEXT ' || pkg.g_context || ' USING PKG';
  3  END;
  4  /

PL/SQL procedure successfully completed

SQL> exec pkg.set_ctx ('a', '123');

PL/SQL procedure successfully completed

SQL> select pkg.get_ctx('a') from dual;

PKG.GET_CTX('A')
--------------------------------------------------------------------------------
123