Network Pools are just like a bucked of isolated networks. If vLAN 30 was an Apartment Building, vCDNI is like taking that apartment building and having 1000 units in it. all isolated places for VMs to communicate.
When you create a new network that needs to be isolated, we basically give it an address which is the VLAN + a secondary number. Kinda like VLAN 30 Unit 20. If this were a postal address, it's like 20-30 VLAN Drive, Phyiscal Network, Your Datacenter, ...
The External network is intended to be the route to the outside world. Real Address that work on the port group you associated it to.
The Edge Gateway is the path between these two. He's the delivery man that goes between the apartment number and the street.
So, you'll use the network pool A) if you create an Organization Network (routed or isolated) OR B) a vApp network. Each creation of one of these networks reduces what's left in the Apartment building by 1, since its occupied.
If you make an Org Network, ANY VM inside that Organization vDC can connect to the network and ping all other VMs in that network.
B) a vApp network is only available to VMs inside the vApp itself.
The thing is, that these two network types have different purposes.
If you could explain the goal ... what I want my network topology to look like, maybe we can help by knowing that.
Big Note ... If you created the Network Pool with vLAN 30 and associated it to a Distributed Switch. VLAN 30 MUST MUST MUST be trunked to all uplinks of all ESXi hosts on that distributed switch.