Both are executed by PRPC agent internally. Queue-For-Execution is used mainly by Service Request Processor. Queue-For-Agent method is available to queue any task to be executed at the back end by Agent.
Pls look into all the links I shared and all inter links. You will find all explanation on Asynch execution.
I kept execution mode as 'queue for execution' in service file and I have service activity and Final activity defined in it. Im running the service file standalone. My service activity is executed but not the final activity. Verified by placing the logs. Am I missing anything here.