What is the use of the ActiveMQResourceManager?

Issue

I have seen from the comments in the class1 itself that:

/**
  * This class allows wiring the ActiveMQ broker and the Geronimo transaction manager
  * in a way that will allow the transaction manager to correctly recover XA transactions.
  **/

What does "correctly recover XA transactions" mean precisely? What happens if this declaration is not present (& for at least one of my resources)? If this is a mandatory declaration, what is the goal of using an XaPooledConnectionFactory, performing the auto enlistment with Aries? What I am trying to gather here is a more precise understanding of the system so that I can extrapolate this to other resources (mainly WMQ JMS and Oracle Datasources in my case).