I think the bottom two can be taplights/dummies on the one I'm building.
It take 7 encoded glyphs (each "locked" by a chevron) to open a Stargate to an address (second gate) within your home galaxy.
An 8th one is required for intergalactic coordinates.
9th is for???? OOH -- plot device! plot device! Maybe 9 is for deliberate transtemporal/transdimensional/multivers