Reach is counting the number of relationships between Activities and Businesses. If an Activity has 10 Businesses linked, it has a Reach of 10. If there are three Activities with 10 Businesses linked to each then the total Reach of those Activities is 30. In effect, it's the number of individual units of value created for Businesses in the BID area.
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​Unique counts the number of unique Businesses linked to Activities. For example, if the three Activities described above were for the same 10 Businesses each time, then Unique will be 10.