The NAT ID is used to associate or bind hosts/subnet from higher security to lower security segment.
For example, to allow internal hosts/subnet to access internet, you need to define NAT and associate it with outside IP or range of Public IP via 'global' command. The NAT and Global ID must be identical, outherwise, no outbound access is allowed.
If no nat with ID of 10 or 30 is defined, then the "global (outside) 10" or 30 is not in used or relevant.
