They are placeholders, their meanings are spelled out in the post:

    O: publish/oembed_cache, empty content, stale timestamp with parent C
    C: future/customize_changeset, changeset JSON with parent C
    P: draft/page, with parent D
    D: parse/request with itself as its parent
    S: publish/post, for providing embed data
    T: publish/post, containing the outer embed

That doesn't actually answer any of my questions though. Why is `S` the placeholder for "post"? T for T'outer?