When a Commander or Squad leader gives a player an order to "Guard this unit", which typically means "build this structure", it does not dissipate once the order has been fulfilled. It remains active until you manually cancel the order via the F menu. There may be other cases where this occurs with other similar commands.
Guard order means guard order, it's just used as the build order too. So even if it's built your still assumed to be guarding it right? What might be better is to have it be a "build this" order if you target unbuilt things.
I agree that there should be a distinct separation of the two. Anyway, most of the time the Commander wants you to build something, not actually guard it, but that's what we have to work with.
guard/build is basicly the same. Its more like PAY ATTENTION TO THIS FRIENDLY UNIT. PAY ATTENTION TO THIS ENEMY UNIT Thats why I use commands for. Unless im spamming 1 target over and over then I want you to really do something