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
> Swap is not generally about getting emergency memory, it's about making memory reclamation egalitarian and efficient. In fact, using it as "emergency memory" is generally actively harmful.
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