As my Mexican-born, US immigrant father would say when my US university educated Irish-born mother would correct his grammar, "I don't speak English, I speak American".
FoS is "Factor of Safety". It is a factor commonly applied to analytical procedures to compensate for inaccuracies that can result from the simplifications used to expedite the calculations. "ksi" is just shorthand for thousands of pounds per square inch.
Strange as it might sound, 25 years ago when I went through college, all of my class work was done in metric (SI) units. But when I went to work in the US aerospace industry, everything was done in "Imperial units" (ie. inches, pounds, etc.). To this day it still is. Most every other US industry (such as automobiles, consumer products, etc.) now uses SI units, but US aerospace still uses inches and pounds.
I'm happy to work in either system. It's just numbers, and one system is really no better than the other. But like most things, it boils down to majority rule. The ground rules are determined by the major players. That's why the standard currency of global trade is presently the US dollar, crude oil is traded in barrels, gold is traded in ounces, and natural gas is traded in Btus.
Didn't mean to confuse or insult anyone, it's just the way my brain works.