Is there a number to be attributed to concentric damage, or at least an explanation to better understand it?
Im not sure what you mean by concentric damage. Can you give an example or a screenshot?
Do you just mean “Near” attacks, that hit your target for standard damage and nearby enemies for “minor” damage? If so, I believe that “minor” just means half damage of the primary hit. At least, that was my initial observation, I hadn’t really done a lot of further investigation after I reached that conclusion. (If I’m wrong, somebody please correct me!)
There are some heroes (such as Evelyn, but there are others) who have Near attacks that do not say “minor” for the sides, and those pack a wallop and hit the sides for full damage. Noice! 
That symbol means an increased chance for Critical hits.
(@Kerridoc @Rook @Coppersky Possible Merge of Topic)
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Please continue the discussion in the linked thread.