Multiple fw4 nft sets in a single rule

Installing and Using OpenWrt
1

Given the following snippet:

config ipset 'rfc4193'
	option name 'rfc4193'
	option enabled '1'
	list match 'dest_net'
	option family 'ipv6'
	list entry 'fd00::/8'

It is translated into the following nftables set, regardless of option match being dest_net or src_net:

table inet fw4 {
	set rfc4193 {
		type ipv6_addr
		flags interval
		auto-merge
		elements = { fd00::/8 }
	}

[...]

}

So, the direction prefix is simply a meta data used internally by fw4. Given the following rule:

config rule
	option name 'Accept ALL from LAN to VPN/RFC 4193'
	option target 'ACCEPT'
	option family 'ipv6'
	list proto 'all'
	option src 'lan'
	option dest 'vpn'
	option ipset 'rfc4193'

If the set rfc4193 is dest_net, the above fw4 rule is translated into:

chain forward_lan {
	ip daddr @rfc4193 counter packets 0 bytes 0 jump accept_to_vpn comment "!fw4: Accept ALL from LAN to VPN/RFC 4193"
}

And if option match is src_net, the rule is translated into:

chain forward_lan {
	ip saddr @rfc4193 counter packets 0 bytes 0 jump accept_to_vpn comment "!fw4: Accept ALL from LAN to VPN/RFC 4193"
}

As you can see, the direction prefix is a meta data, which helps fw4 know to translate the set into daddr or saddr in nftables chains.

What if I want to use 2 sets: one as source and the other as destination? Or maybe the same set as both source and destination? Contrived uci syntax:

config rule
	option name 'Accept ALL from LAN to VPN/RFC 4193'
	option target 'ACCEPT'
	option family 'ipv6'
	list proto 'all'
	option src 'lan'
	option src_set 'rfc4193'
	option dest 'vpn'
	option dest_set 'rfc4193'

I do not know what was the idea of the developer who designed the so called "direction prefix" syntax. Maybe there were some technical difficulties behind it. But it goes against normal intuition.

2

The NFTSET itself does not have a direction but is is for the accompanying rule.
dest_net translates to daddr and source_net to saddr

At least that is what I think
I have made some notes maybe they can come in handy.
See the last Paragraph about NFTSET:
OpenWRT Policy Based Routing (PBR)