To make sure I'm understanding this correctly -- and I'm going to keep my example to Seattle in an attempt to keep it simple -- can I logically conclude that the same would be true for other UCAS A and AA corps? Would this be true of NeoNET as well?
Most people have a national SIN (SR5 p.84). People hired by a megacorp gain a "corporate limited SIN" (p.84) -- these are people who most likely have dual citizenship (national and corporate limited), unless they started SINless. People who were born into a megacorp (or working there when it first gained AA status) have a "corporate born SIN" (p.85). They might never have any other sort of SIN. If they're expelled from their megacorp for some reason, they either revert to a national SIN (depending on circumstances) or become SINless if that's the way the megacorp wants it (see previous posts in this thread).
Note: A double-A rating from the Corporate Court means the corp has extraterritoriality and can officially be called a "megacorporation" (there are dozens of these). A triple-A rating means they
also have at least one of the thirteen seats on the Corporate Court itself (currently, there are only ten AAA megas). Corporations rated single-A and below (the vast majority of companies) do not have extraterritoriality and cannot grant citizenship ... all of their employees have national SINs.
NeoNET is kind of a special case. It's the youngest of the Big Ten, formed in 2064 by a merger between Novatech, Erika, and Transys Neuronet, so there aren't any adults who were actually born into it. Novatech itself formed in 2059, from the ashes of Fuchi. If NeoNET collapses (which seems likely), any or all of the original three might well continue as double-A megacorps, and some of their citizens might have corporate born SINs dating back before the merger, but new hires and internal transfers complicate things. People with a limited Erika SIN might have been Finnish citizens first, Transys Neuronet SINners might also be British subjects, and Novatech employees might easily have been born with Fuchi SINs. Any of them might suddenly find themselves unemployed in Seattle after a NeoNET breakup.