We have a system with a local RADIUS server for people who want to use their prepaid cards for Internet access. But we also have customers who should have access to our system because they're a member of a certain organisation. These records are being kept by another/external RADIUS server.
thats exactly what realms are good for.
it's possible to use "realm" in a more extended view, i.e. <prefix>/<user>@<realm>
where <prefix> and <realm> both could describe a particular realm, customer, organisation unit, etc.