>>What Craig may mean is that there's a development of a new version (say v5.0 of the app) while v4.6.2 is still out there, and some users are still running v4.2 or v3.9. These get their regular updates and bug fixes, and each fix is evaluated by its merits - i.e. whether v5 is affected with the bug, and whether the bug can be fixed the same way in it, or is it time to do the whole thing in a different manner.
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>>Mike Yeager, if he's listening, may tell a more detailed story of how this works - and as far as I remember, it works fine. But then, Siriusware is not a two-man shop.
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>Using your example above I assume there would be 4 versions of source code in all. Now the big question: are each of these maintained (i.e. fix code manually added to each) separately?

Yes. Not just separate projects, but actually separate VSS databases. The changes which needed to be propagated across major versions would be handled by programmer in charge of the issue at hand - by copying and pasting from old into new or vice versa, as appropriate. I wouldn't go deeper here - not because I'd be about to breach any NDA there, but rather because it's been more than a year since, and I'm forgetting the details.