Query/Table term confusion

From: JasonCook (anonymous_at_discussions.microsoft.com)
Date: 03/22/04 

Date: Mon, 22 Mar 2004 13:56:11 -0800

Looking at http://www.databasejournal.com/features/mssql/article.php/3112381 the guy creates a table called Orders, then has the query
select OrderId, OrderDate, O.OrderAmt
        ,(select sum(OrderAmt) from Orders
                          where OrderID <= O.OrderID)
          'Running Total'
from Orders O

My question . . . what is the purpose of the "O"
Why does he specify O.OrderAmt yet doesn't use it anymore until the subquery? Then, why does he use it with O.OrderID in the subquery?

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