Hi Sethupathy,
First of all, this post seems to be related to Adobe Campaign classic, not standard version, right?
If a Community forum admin can move it to ACC section, it would be nice.
You are using webApp page compatibility v5, not v6 page.
But I will try to help you anyhow.
1st question:
Your screenshot displays the advanced setting of a listbox object, with the Context section indicating the Xpath parameters of the query collection name, the id and the labels of such lookup list.
It is not dedicated for a simple variable management.
But why don't you use the standard object Value that displays a variable value, see screenshot below?
It uses by default the variable with its xpath syntax and displays it.
2nd question:
Use the On values changes of Advanced section to propagate changes done elsewhere in your form, to other objects, if dynamically refresh must be done.
Otherwise, simply use it in Script element of your web activity page v5, instead JS activity in the webApp (otherwise do a loop to come back into the web page with the external treatment or checks done).