For userstate information, these are the docs you’re after https://dev.twitch.tv/docs/irc/tags/
And method, that’s just a property that the module adds to say what type of sub it is, all of which can be found in userstate anyway.
Also keep in mind tmi.js is heavily out of date. Last time I checked it hadn’t been updated in months, so didn’t include any of the gift sub stuff.