Цитата https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-xp/bb490982(v=technet.10)

Redirecting command input (<)
To redirect command input from the keyboard to a file or device, use the < operator. For example, to get the command input for the sort command from File.txt:

The contents of File.txt appear in the Command Prompt window as an alphabetized list.

The < operator opens the specified file name with read-only access. As a result, you cannot write to the file when you use this operator. For example, if you start a program with <&2, all attempts to read handle 0 fail because handle 2 is initially opened with write-only access.

Note

Zero is the default handle for the < redirection input operator.