Chain
Every chain switch our node recorded, newest first. These are node-local observations: an absent event is absence of observation, not proof that no fork existed.
Chain switches recorded by our node, newest first.
The unit is recorded reorg events, not blocks: each row is one switch our node made from the chain ending in the old tip to the chain ending in the new tip. The list is what our node saw, not an exhaustive record of every fork on the network. The losing tip is retained as a non-canonical block, never deleted.