In the case of the captive portal I made for a local cafe, the "Guest Network" allows a certain range of IP addresses. This would contain the web page/app that lets you "sign in" to the AP. After signing into the AP you now have a different range of IP addresses you can access (or be denied from!)
I used Unifi in my implementation. The interface to configure this via unity can be seen here (I just binged "unifi guest" this is not my config)
Thanks damian! This was a good explanation!
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