The downtilt omni creates a donut like coverage with more bias towards pushing the signal downwards than the standard omni panel.
Coverage patterns can be found in the datasheets.
C5
D5
General guidance is to use the downtilt for mounting heights between 8-25 feet. And generally this would mean the antenna is mounted parallel to the floor (pointing down). You could mount the antenna parallel to a wall understanding that you'll be sending a lot of signal up & down.
You don't often see an internal antenna AP mounted on a wall for the same reason. It's shooting a lot of the signal into the ceiling and floor instead of outward to clients below.
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