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haveo
Probably for holding the hub while you try to remove sticky sprockets, so that you don't have to resist the torque by holding the rim and thereby rip the hub free from its moorings :-)
There doesn't appear to be a left-hand lock ring thread, so the notch may be for a right-hand threaded lock ring with some sort of detent mechanism.
It could be a jigging point used during assembly which has no purpose for the end user.