An internal license is provided by a "partner noc", a hosting company that hosts the server where cpanel is installed and provides de license. You are only allowed to use that server inside the partner noc.
An external license is good if you host your server at a datacenter that doesn't support cpanel or if you host it yourself or colocate it somewhere.
If your host/colo/dedicated provider is signed-up with CPanel as a partner NOC or distributor, their IP addresses are "registered" for internal license pricing. This means they get a significant discount on licenses that are signed-up using any of their "registered" set of IP addresses.
External licenses are pretty much "set-rate" at about $55/mo w/o support, and can be used anywhere.
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