Sending Atlas requirements to many backlog projects (e.g. JIRA)

Atlas help says that one a project can be set up so that requirements from one Atlas project can be sent to multiple projects (backlog) through use of a UDA.

For instance if we are using JIRA as our external issue tracker, is this accomplished by creating a drop down list UDA within Atlas that has values that will be used to identify the different options for backlog projects (i.e. JIRA projects) and then within Connect how is the mapping defined?  Is one Hub project mapped to multiple JIRA projects and how is the criteria set up to determine which backlog item goes to which project.

 

Are problems introduced if the UDA value on the requirement is changed after a story has already been sent to the backlog from that requirement?