Open Stack

Hello everyone, I am trying to figure out a few things in quantum the admin guide says that there are 5 models

The first says
	Single NIC etc etc.. and it does not support Floating IP? Why? (single Flat Network)
the second says same as the first but which network to attach to which is pretty clear. 
the third says enables multiple NIC, routing NAT and LB (Mixed Flat and Private Networks)
the fourth says supports floating IP address isolated network using SNAT rules on external router and no overlapping IP address (Provider Router with Private Networks) 
the fifth says Overlapping IP tenant can create their own routers etc.. 

so to resume why single cannot use floating? What will be the case and when should I use one of the others instead of using the Per-Tenant Router with Private Networks?

Thanks
Remo