When using the "standalone" mode you can use the command-line flag "-c" to select which configuration file (aka "profile" in domain mode) you want to use. You can create more profiles by just copying one of the existing files "standalone.xml", "standalone-ha.xml" etc.
The port offset does exactly what its name implies: You can use this parameter to add its value to the predefined port numbers:
By default the management console will listen on port 9990 and workload will run on 8080. If you specify an offset of 10000 (-Djboss.socket.binding.port-offset=10000) management will lsiten on 19990 and your applications will be reachable on port 18080.
port-offset is typically used to make it possible to run more than one server on the same host and interface.
You'll find the detailed documentation including configuration examples in the reference documentation (https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.2/)
There is also http://www.mastertheboss.com with lots of configuration examples ...