My System Study Group Chapter 2: Open Files
Open files. Nimzo gives the following position, and makes the point that a file is open when your pawns do not block your Rooks or Queen from attacking on those files. He also makes a point that the piece attacks the squares of that file with or without the opponent having a pawn on that square. He also says that the occupation of the 7th and 8th rank is the goal. That you can do this by way of an undefended square / pawn or my massing an attack on the square / pawn. As long as it is attacked one more time than it is defended.
Example: Opening a file.
Example: Thomas vs Alekhine.
Example: Gaining the back ranks.
Example: Evolutionary and Revolutionary attacks.
Example: Restricted Advance in a File
Example: Advance posting.
Example: Nimzowitsch - Amateur.