In the documentation for /filter the watching_first_post filter is mentioned.
However, this does not seem to do anything.
It’s also not available as a regular ?state= parameter, while watching etc are.
Looking at the code it seems that it uses TopicUser and I don’t think that works for watching_first_post if I remember correctly?
I had the same question when the documentation was published, too.
But I am not sure if anything has changed since then.
But for a topic list it does make sense?
It should display “topics from categories where you watch first post only”.
Thanks for the report @RGJ 
Both “filters” should be fixed by
in:watching_first_post - Show topics you’re watching for first post
Does it make sense to also adjust the text shown? Something like ‘Show topics from categories and with tags you are watching for first post’?
Then it might be a little easier to understand that tracking, watching, normal and muted refer to what you have selected for the topic, but first post ultimately displays topics with all these statuses based on category and tags. It would then also be clear that the tracking and watching filter are not only based on categories and tags where this has been set as the default, but also to all topics where the status is the same for other reasons. For example, because you created the topic and are therefore watching it.
Sure, that makes sense
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