The Resource Loader is a facility that allows easy configuration and management of resources. Pools configured via the Resource Loader will be created automatically during the transaction manager's startup and will be closed during its shutdown.
It consists of an extra layer above the JDBC and JMS pools which configures them via a properties file. It is not mandatory to use it but sometimes it is better than manually creating resource pools. Another good example of Resource Loader usage is when BTM is embedded in a servlet container which configuration file cannot easily be extended, like Tomcat for instance.
This document has not been updated for version 1.3.x yet but you can still read the 1.2 version as most of its content is relevant for both versions.