Epoch ms at which the shard was stored; drives oldest-first draining and retention.
The MJ user whose call this is, or null when the page had no signed-in user. A page only lists (and so re-sends) its own user's shards, and logout deletes them: on a shared browser one user's audio is never sent with, or left behind for, another user's session.
A shard waiting in the store for the server to acknowledge it.