What is --admin-only mode in JBoss EAP?

Issue

• How do I start JBoss EAP in admin-only mode?
• Need to understand the usage of --admin-only and also how to identify if JBoss server is running in --admin-only mode?
• How to update the standalone/configuration/*.xml file without starting the server?
• How to start the server back in NORMAL mode via CLI?
• Unable to run JBoss CLI command /subsystem=datasources/data-source=ExampleDS:test-connection-in-pool and getting an error which I assume is unavailable in admin the only mode:

 {
     "outcome" => "failed",
     "failure-description" => "JBAS014884: No operation named 'test-connection-in-pool' exists at address [
     (\"subsystem\" => \"datasources\"),
     (\"xa-data-source\" => \"oracleDSxa\")
 ]",
     "rolled-back" => true
 }