JPA implementations have the choice of managing transactions themselves (
RESOURCE_LOCAL), or having them managed by the application server's JTA implementation. In most cases, RESOURCE_LOCAL is fine. This would use basic JDBC-level transactions. The downside is that the transaction is local to the JPA persistence unit, so if you want a transaction that spans multiple persistence units (or other databases), then RESOURCE_LOCAL may not be good enough.
JTA is also used for managing transactions across systems like JMS and JCA, but that's fairly exotic usage for most of us. To use JTA, you need support for it in your application server, and also support from the JDBC driver.
In your current configuration, its JTA type datasource.In a Java EE environment, if this element is not specified, the default is JTA.