[1] factor first appeared on 5th edition Research Unix in 1974, as a "user maintained" utility (section 6 of the manual).
In some variants of Unix, it is classified as a "game" more than a serious utility, and therefore documented in section 6.
[3] A free software version of the factor utility was written for the GNU project by Paul Rubin, in 1986.
The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.
Various projects, including simple ones such as printing prime numbers, are facilitated by using this command.