When I saw that, the first thing that came to mind was that it's the military occupation specialty (MOS) for US Army special forces (18x).
https://en.wikipedia.org/wiki/United_States_Army_Special_For...
Am I naive? How is "18F" inappropriate?
https://en.wikipedia.org/wiki/United_States_Army_Special_For...
Our name is short for the address of the GSA building where we’re headquartered in Washington, DC: 1800 F Street.
For those curious about the inappropriate-sounding name