Basically Ajax is using Ajax requests when you page/sort/group/filter also Ajax request when you add/update/delete records.
In Server mode each operation is doing a GET or POST request to the server and it refreshes the whole page. However when using server binding you have better search indexing (like google etc) you can give url-s which represent the current Grid set.
In Ajax mode the user interaction will be smoother and the server will not have to re-create the whole page - which makes it the lightweight scenario.
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