in one Trigger, other "Sold Units" from other triggers won't be added to that variable? I mean, only Sold Units from that trigger will be added to that variable array...? (I've had that question for over a week xD) I quite don't get what you wanted to say there :S If you have other trigger that refers to exact same event (Sells unit) just add conditions in both so only what given conditions match unit array is set, thats why conditions are here.
About group - I have just mentioned adding heroes into global group because it can be usefull for further purposes in your map - thats all.