Package STANDARD:

The package named STANDARD describes the PL/SQL environment. A package specification worldwide declares types, subprograms, and exceptions that are available automatically to every PL/SQL program. For instance, package STANDARD declares the subsequent built-in function named ABS that returns the absolute value of its argument:

FUNCTION ABS (n NUMBER) RETURN NUMBER;

The contents of package STANDARD are straightly visible to applications. Then, can be called ABS from a database trigger, an Oracle Precompiler application, a stored subprogram, an OCI application and several Oracle tools involving Oracle Reports, Oracle Forms and SQL*Plus.

If ABS is redeclared in a PL/SQL program is a local declaration overrides the global declaration. Though,   the built-in function can still be called through using dot notation, which is given as follows:

... STANDARD.ABS(x)...

Most built-in functions are overloaded. For instance, package STANDARD holds the following declarations:

FUNCTION TO_CHAR (right DATE) RETURN VARCHAR2; # function declaration#

FUNCTION TO_CHAR (left NUMBER) RETURN VARCHAR2;

FUNCTION TO_CHAR (left DATE, right VARCHAR2) RETURN VARCHAR2;

FUNCTION TO_CHAR (left NUMBER, right VARCHAR2) RETURN VARCHAR2;

PL/SQL resolves a call to TO_CHAR through matching the number and datatypes of the actual and formal parameters.