It was about encryption ciphers, when the US had export restrictions on key lengths. U = USA = 128bit, I = International = 40bit, N = None. Nowadays the U is another vestigal piece of the UA string.

Quick search answered my own question - 'U' indicates USA; As a result of cryptographic export restrictions, different levels of security were shipped in early browsers: U(SA) = 128bit, I(nternational) = 40bit, or N(one).

It first showed up in Netscape 1.x.

Getting it removed from Gecko was https://bugzilla.mozilla.org/show_bug.cgi?id=572668 . Chrome and Safari followed.

"Strong security (Default) the browser provides crypto support that is stronger than what the "international" builds of Netscape offered circa 1995."

https://cat-in-136.github.io/2010/08/u-in-ua-string-and-abou...