Prefix_filter for odhcpd

I'm trying to understand how the option prefix_filter is supposed to work in /etc/config/dhcp. I have the following ULA in /etc/config/network:

# uci show network.globals
network.globals.ula_prefix 'fd00:beef::/48'

If I wanted RA to advertise just that ULA prefix (and not other prefixes from ISP) to internal computers, is prefix_filter supposed to control RA with the following entry?

# uci show dhcp.lan.prefix_filter
dhcp.lan.prefix_filter 'fd00:beef::/48'

I'd usually use ip6class for that, but I have no idea how to use it for ULA prefixes.

Neither do I because the option value 'globals' don't work with ip6class. That being said, I don't want to limit br-lan to ULA only because I'll like to allow static IPv6 machines to continue using public prefixes.

In the LuCI settings, you tell the interface only to issue prefixes from "local." You will only be issued the ULA...

You also know, you don't have to auto assign prefixes, you can disable that anyways.