SYS_GUID()

In Oracle PL/SQL, SYS_GUID is a built in function which returns the Global Unique Identifier (GUID) for a row in a table.  It accepts no arguments and returns a RAW value of 16 bytes.  A GUID is a sequence of characters that are supposed to be globally unique. In other words, they should never appear more than once regardless of the circumstances. You would normally use GUIDs when you need absolutely unique values that involve multiple database instances or multiple networks. The algorithm that Oracle uses is somewhat faulty in that the values are unique but they are not random (or even pseudo-random).
Source: http://psoug.org/definition/SYS_GUID.htm

Example:

select sys_guid() from dual;  evaluates to 717A66D4915946D59152CC7FD4F517CC (in this instance).

select rawtohex(sys_guid()) from dual; evaluates to CCDE794E51F8442F99505E5EFF83FCA3 (in this instance).

One place this can be useful is in the generation of CCR, CCD, or CDA documents, where an unique identifier is required within the XML.

Another place this can be useful is as a default value for a primary key column.  However, I prefer to use a sequence and a table trigger, so the primary keys are nice, sequential and clean.