This is from the NAT RCF:
The following is a description of the properties of realms supported by Twice-NAT. Network address of hosts in external network are unique in external networks, but not within private network. Likewise, the network address of hosts in private network are unique only within the private network. In other words, the address space used in private network to locate hosts in private and public networks is unrelated to the address space used in public network to locate hosts in private and public networks. Twice NAT would not be allowed to advertise local networks to the external network or vice versa.
I'm confused about the last sentence, why can't the external network be advertised to the private network? Is it because
Network address of hosts in external network are unique in external networks, but not within private network.
Meaning that the private network might be using an external network address?