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Hello thank you for using JA let me see if I can help.
Is this a new tire?
Is it located above the valve stem for the air?
That is the indicator from the maker of the tire that tells the person installing the tire to align that mark to the valve stem for the best balance on the tire. Most people do some do not it really does not effect anything since the balance machines used know are really accurate and unless you feel a vibration I would not worry about it.