Maybe this should be documented here: Configure webhooks that trigger on Discourse events to integrate with external services.
You can get the post types from the rails console by running Post.types. That returns
:regular=>1, :moderator_action=>2, :small_action=>3, :whisper=>4
Any regular post on the site will have its post_type field set to 1 (regular.)
Post type 2 (moderator_actions) is for posts that are generated when moderators handle flags in the review queue. I think that’s the only time that post type is used.
Post type 3 (small_actions) is for posts that are automatically generated when a topic is opened, closed, etc.
Post type 4 is for whisper posts.