AJAX is generally used on the client side, not usually the server side.
It sends requests to a server.
You probably need to provide more information about what you are trying to achieve to get a decent answer.
If you are wanting to transfer data from a “client” to a server then you could send the data in a variety of ways – especially with AJAX. Just how depends on what data type and size. Requests from a client to server (HTTP) can include POST data, which can carry your data to the server.
If you are using AJAX on a client and wanting to use it to transfer data from one remote server to another then you could get AJAX on your client to call a script on the server that would instigate the transfer from the server to another server in some way. (such as FTP or SCP maybe – or even using a server to server HTTP POST)
If you can be a little more specific about what you need to do then I will try and give you a better answer.
solved Server to server transfer using AJAX