please disregard and see the next post. thank you
In the jboss-service.xml descriptors we simply inject values to any number of setable attributes, we don't declare the attributes there.
If we are talking about standard mbeans, attributes are derived from the MBean interface implemented by the class (using reflection), by those methods that follow the java bean pattern:
public <some-type> getAttrName();
public void setAttrName(<some-type>);
(read the jmx spec).
In jboss we also have xmbean (when an xmbean descriptor is provided, where we can explicitly specify the attributes there).
See for example the descriptors in server/default/conf/xmdesc directory.