Comment about "WAL decoder slots"

[hasegeli]

I am confused about the comment in here:

pg_failover_slots/pg_failover_slots.c

Lines 911 to 928 in e26870b

	/*
	* Do not synchronize WAL decoder slots on a physical standy.
	*
	* WAL decoder slots are used to produce LCRs. These LCRs are not
	* synchronized on a physical standby after initial backup and hence are
	* not included in the base backup. Thus WAL decoder slots, if synchronized
	* on physical standby, do not reflect the status of LCR directory as they
	* do on primary.
	*
	* There are other slots whose WAL senders use LCRs. These other slots are
	* synchronized and used after promotion. Since the WAL decoder slots are
	* ahead of these other slots, the WAL decoder when started after promotion
	* might miss LCRs required by WAL senders of the other slots. This would
	* cause data inconsistency after promotion.
	*
	* Hence do not synchronize WAL decoder slot. Those will be created after
	* promotion
	*/

What does it mean by "WAL decoder slots"?