Zyxel Vlan "Base Ports"

Hey r/

I'm busy with testing VLAN setups on Zyxel Firewalls since my company has no experience with VLAN's.

I noticed that there is a "Base Port" setting. What does this mean?

I've set the "zone" to LAN1 so I assume that traffic for VLAN10(in this case) is going to LAN1.
The "Base Port" is by default set to WAN1. Is this meant to indicate the interface to the internet?