5733-XT2 replaces 5733-XT1 from V5R3 and V5R4. The XML Toolkit actually preceded any built in support in RPG that came with V5R4. (COBOL had support starting in V5R3.)
The XML Toolkit provides alternative APIs for the native ILE RPG (or COBOL) support.
Some introduction can be found at XML on IBM i and XML Toolkit (XT2) Description/Installation. A short mention of how the Toolkit might be useful outside of technical programming can be read at DB2 UDB Extenders for iSeries, XML Extender requirements.
Some detail of how the Toolkit can be used in RPG is in the IBM Systems magazine article XML Toolkit for iSeries Provides Interface for RPG and XML.
I would classify it as a programming tool(kit).