What do I do with INTERFACE WAN_SLAAC GATEWAY?

[opensensical]

I am trying to set up IPV6 on dual-WAN setup.
My ISP said to enable SLAAC mode, which I did in opnsense.
Then, INTERFACE WAN_SLAAC GATEWAY appeared in my list of interfaces when setting up groups.
It does not appear in INTERFACES list in opnsense.
I am setting up my IPV6 groups similar to my IPV4 groups.
I have one group for load balancing and one for failover.
I notice the WAN_SLAAC GATEWAY is set to NEVER. Am I supposed to change that?
Why did enabling SLAAC give me another gateway? Am I supposed to configure it some way?
There is nothing in the docs about this.