Can anyone explain the reasoning behind the location requirements for 90+ furnace vents?

1) Why can a two-pipe (intake and exhaust) vent be within 12 inches of a window, but a single (exhaust only) vent can't be within 4 FEET? I assume it has something to do with the exhaust air pressure??

2) Why can a two-pipe (intake and exhaust) vent be within 12 inches of an OPENABLE window, but must be 4 FEET from a FIXED, NON-OPENABLE window? I'll admit, I can't even come up with a theory on this one...

3) Where can I find "highest anticipated snow levels" for my area?

Thanks so much,
Brian