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Each column shows that channel's full picture — its own calendar plus the days your calendar sync closes there (read from the other channels' feeds). Hover a day for details. The check-out day of a stay stays open, as on the channel itself.
Closed here — this channel's own calendar publishes the day as closed (its own reservation or a manual close). Past days that the feed no longer publishes are kept from the internal registry of observed reservations.
Closed by another platform — not in this channel's own feed, but booked/closed on another channel: your sync closes it here. (Channel public feeds don't export synced closes, so these are read from the other channels.)
Open — free on this channel and on every other channel.