I've never fooled around with vol much but just did some simple subtraction and Advancing minus Declining equals the nasdaq net $ADDQ. It follows that
+1 for 1 advance. -1 for 1 decline. 0 for 1 unchanged.
equals the same without considering the zero for unchanged issues. Essentially each type is assigned a weight from -1 to +1 with unchanged issues assigned a weight of zero.