Understanding the archive_cleanup_command In the postgresql.conf file for PostgreSQL version 13, the archive_cleanup_command comment explains the command in the following way:

#archive_cleanup_command = ''         # command to execute at every restartpoint.

The documentation here and here have no mention of a 'restartpoint'. This raises the following questions:

What is a restartpoint?
For example: is restartpoint just the same word for a checkpoint? Do the two mean the exact same thing?
When is a restartpoint created?
For example: if the restartpoint is just a checkpoint then the check point will be created every 5mins or whatever the setting for checkpoint_timeout is in postgresql.conf file.
When is the archive cleanup command run?
For example: The archive cleanup command is run every time the archive_timeout (set in the postgresql.conf file) is reached. If the archive timeout is set to 1hr, then the archive_cleanup_command runs every 1hr.
A good way to find such information is to go to the index of the manual and look for &quot;restartpoint&quot;. Which indeed is listed there and leads to WAL configuration

In archive recovery or standby mode, the server periodically performs restartpoints, which are similar to checkpoints in normal operation: the server forces all its state to disk, updates the pg_control file to indicate that the already-processed WAL data need not be scanned again, and then recycles any old log segment files in the pg_wal directory. Restartpoints can't be performed more frequently than checkpoints in the master because restartpoints can only be performed at checkpoint records. A restartpoint is triggered when a checkpoint record is reached if at least checkpoint_timeout seconds have passed since the last restartpoint, or if WAL size is about to exceed max_wal_size. However, because of limitations on when a restartpoint can be performed, max_wal_size is often exceeded during recovery, by up to one checkpoint cycle's worth of WAL. (max_wal_size is never a hard limit anyway, so you should always leave plenty of headroom to avoid running out of disk space.)