You just need to add one criteria to the CASE statement. Like this:

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UPDATE #Counter2

SET @Counter = Counter = CASE WHEN (@Counter >= 1 AND @Counter < 10) OR ([Value] >= 10 AND [Value] <=15 AND Code = 'a')

                           THEN @Counter + 1

                           ELSE 0 -- restart the counter

                           END

   ,@RowNumber = RowNumber

FROM #Counter2

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