Indeed the custom validation is only on the client-side. It provides a way to extend the built-in rules with custom ones like the demo example we have for the ProductName starting with a capital letter. This is checked on the client and is triggered as soon as the input is blurred.
In contrast the remote validation is also a custom rule so to speak, however, it posts the values to the server with an ajax request and in accordance with the response - the grid displays the valid or invalid state, but the validation is performed on the server side, most frequently used for checking for duplicates in an existing database or other information that cannot be inferred without a check with the server side.
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