I think in the case of Manual Mode, you can't use addresses which are assigned elsewhere.
e.g. you have an IP Pool for the external that is 3 addresses, so those can be programatically used else where when creating VMs. So vCloud Director manages these 3 addreses automatically. Therefore, don't manually use them somewhere else because it could (theoretically) cause a duplicate.
In the case of automatic NAT, vCD can draw from the existing pool of IP addresses and assign exactly 1 for the VM in question to be routed through for IP translation.