Life of a post
A post that already published, is publishing, partially published, or failed cannot be edited.
Each network on the post has its own status (pending, publishing, retrying, published, failed), plus a public URL and an error after the attempt.
A time on create is just stored. The post stays a draft until you actually schedule or publish.
Per-network caption and media
Most of the time every network gets the shared caption and files. When LinkedIn needs a longer version, or Instagram should go out without the video, you override that row.
Length and media rules use what that network will actually receive. A long shared caption is fine if the overflowing network has a short custom one.
Settings that only apply to one network
Some networks need extra fields before they will accept a publish: a Pinterest board, a TikTok privacy level, a Discord channel. Those live next to the caption, not inside a generic blob. Drafts are not checked. The check runs when you schedule or publish. Full key list: Per-platform meta.Formats
There is no live REST catalog of formats. Use the platform pages or, from an agent,list-content-types-tool.
There is no LinkedIn carousel format and no Instagram carousel format. A LinkedIn PDF is a regular post with a document. An Instagram carousel is a feed post with several images.