File (formation)
Military unit of troops aligned one behind the other
From Wikipedia, the free encyclopedia
In military organization, a file is a number of troops drawn up in line ahead (i.e. one behind the other) in a column. The number of files is the measure of the width of a column of troops in several ranks one behind the other.[1][2][3][4]

Usage
Files are useful when troops don't know where the enemy is, since there are overlapping fields of fire from each soldier, and cover from a possible flanking attack. Files are at a disadvantage when there are heavy weapons nearby, supported by infantry, especially machine guns and tanks.[1][2]