How is the Call made when click on link is it AJAX?

I guess it does not do AJAX calls most of the time. Based on my understanding

With each interaction, the entire SAIL expression must be evaluated to tell the client what has changed, including all the interfaces, rules, and functions it calls. This evaluation must happen no matter how small the change in what is displayed.

With each interaction, the entire SAIL expression must be evaluated to tell the client what has changed, including all the interfaces, rules, and functions it calls. This evaluation must happen no matter how small the change in what is displayed.

Every time the user interacts with a component, the client sends the server the new component value. The client then waits for the server to respond and then renders the resulting interface.

To get the better understanding please read the below link

https://docs.appian.com/suite/help/17.2/SAIL_Performance.html

Most web now is ajax. With SAIL Appian makes it to next level. The over interaction is described here docs.appian.com/.../SAIL_Performance.html and what all on a pages gets refreshed depends on how you configured the page using load and with functions.

You can pretty easily see what happens on the client side by hitting F12 in your browser and watching the network trace when you click on an item in the form. There's ultimately a server round trip for every interaction.