Inconsistent Syntax around Windows by Allan Mitchell

When creating any kind of window over your streams you have Input and Output policies. Output policies have to be specified and Input are optional.
Both policy types have only one option available for each type of window so why enforce the Output Policy and not the Input Policy. I am willing to bet that most people do not even know an Input policy exists

http://msdn.microsoft.com/en-us/library/ee842704.aspx