The difference is:

You are setting your instance name locally (temporarily for that session) thru two_task parameter and connecting to SQL*Plus. So , it defaults to what you have set in TWO_TASK parameter.

You care connecting to whatever the service_name/database you wanted by explicitly specifying @xyz.

[url]http://www.dbasupport.com/forums/showthread.php?threadid=6208[/url]