River Device functions

I’m having trouble discerning the function of the Global Commands and Display Overlay options.

Could someone explain the functions of these?

I haven’t used manual rivers since forever, but overlay options basically let you choose “what kind of data” do you want your “viewport overlay animation” to show. For example, if you choose “flow direction”, the river overlay will show colored gradients moving in the flow direction. You need to increase your “bankful width” to see the overlay in action properly. Or zoom in very close to your river.

As for global commands, it just has this one option. It probably does what the button says, “set all rivers to terrain”.