Function changed 

async fn changed(
    services: &Services,
    sender_user: &UserId,
    since: u64,
    next_batch: u64,
    requested: &[ProfileFieldName],
) -> Result<Candidates>

The fields changed in (since, next_batch] that this client may see.

The log is read under the syncing user’s own prefix and under each room they have joined, because every write is recorded under both. Their own changes are a MUST so that their other devices learn of them, and the rooms are the whole joined set rather than the rooms this response carries: a member’s new status matters to a client whose room had no events.