A file is a military term for 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.
Refer to Rank (Formation).
Files are useful when troops do not 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.
Ancient Greek Use
A file of men in the Greek phalanx was called a lochos (Greek: λόχος) and usually ranged from eight to sixteen men.