Nftables conntrack state matching JSON syntax

[Mikołaj Pisula <mik.pisula@xxxxxxxxx>]

I have these two rules in my firewall:

ct state invalid drop
ct state { established, related } accept

When I use this command to get their JSON representations, i get the
following result:

# nft -j list ruleset | jq '.nftables[] | select(.rule.expr[]? and
.rule.expr[].match.left.ct?) | .rule.expr[0].match'
{
 "op": "in",
 "left": {
   "ct": {
     "key": "state"
   }
 },
 "right": "invalid"
}
{
 "op": "==",
 "left": {
   "ct": {
     "key": "state"
   }
 },
 "right": {
   "set": [
     "established",
     "related"
   ]
 }
}

Matching the "state" key with "invalid" uses the "in" operator, while
matching that same key with a set of "{ established, related }" uses
the "==" operator. Why is this the case? According to
libnftables-json(5) the "in" operator is used to "Perform a lookup,
i.e. test if bits on RHS are contained in LHS value." Is this the
case? If so, could I ask what the bitmasks for these states are?

Thank you for your time.