Line of Site using Flood Routes

I think it depends on what coverage you are going to see from your opponent.

If you know you are going to get Cover 3/4 then I would tell the QB to read the Flat defender from the 12 yd. Out to the Flat route. The QB should be "Alert" to the Fade on the outside and the BS Post. Against Cover 2/1/0 I would tell the QB to read the CB from the Fade to the 12 yd. Out and be "Alert" to the BS Post while having the Flat route as the outlet.

When you say "Alert Backside Post" to the QB then he would know to look at that route either first or second in his progression. For example, if a Safety likes to cheat on this play then the "Alert" call will tell the QB to look at the Fade first to pull the Safety over and then throw the Backside Post.