So the routing algorithm works like this. There's something called the contention window. And the very first part is for routers and repeaters. And they will always broadcast anything they hear no matter what. The second part of the contention window is reserved for clients to rebroadcast any transmission that they only heard one time and the farthest away node by signal strength is the one that rebroadcasts it. The third part of the contention window is used by what are called router late nodes, which are meant to fill in coverage gaps. They will always rebroadcast whatever they hear, but they will only do so after all routers have already broadcasted, and after all clients have already broadcasted, if needed. By using router mode at low elevation you cause something called hop gobbling where the router will broadcast the message and not end up letting it broadcast to a node that would be better positioned to broadcast it farther. 100 feet isn't a hard and fast rule, but it's a good general idea because the higher the node is, the better it would work as a router. A node that is, say, 50 feet on a tower on top of a mountain would be a great router, even though it's only 50 feet above the ground. It's way above the valleys around the mountain, so it would work fine. If you live in a flat land, the absolute highest towers would be the best place for routers. Generally, if you have access to such a high spot, you will probably want to coordinate with other local mesh users in order to optimize the placement of your nodes. Also, since routers broadcast as soon as they hear a message, a badly placed router can actually decrease the distance a message can travel across the mesh and therefore hurts everybody.