My personal theory is . . .

6 Chevrons: ID a point in a galaxy.
7 Chevron: Origin point.
8 Chevron: Galaxy ID, defaults to current if not used.
9 Chevron: Override.

That is Chevrons 1-8 are used in normal use to dial a system ID and it picks up the main system gate or any active gate in that area if the main gate is down. The 9th Chevron tells the system instead to dial a single specific gate code made using the other 8 Chevrons.

So your in the milky way and dial a 7 chevron earth code you get the SG gate as its the main gate for earth or if that's down the back up gate for that system. If you dial 1-9 your telling it to dial specifically the main gate or the back up gate depending on which gate code you enter.

The 9th Chevron wasn't orgin/target/speed/etc it was an override telling the system rather than using the usual target/origin point system to instead dial the Destiny gate direct and if for the sake of argument Destiny had 2 gates in it and the one for that code was destroyed nothing would have happened. It was needed for destiny because it was a far more variable target location but it could work for any gate if you had that gates' 8 chevron ID.