Replication slots were introduced with PostgreSQL 9.4 and are designed to ensure that any standby connected to the primary using a replication slot will always be able to retrieve the required WAL files. This removes the need to manually manage WAL file retention by estimating the number of WAL files that need to be maintained on the primary using wal_keep_segments (PostgreSQL 13 and later: wal_keep_size). Do however be aware that if a standby is disconnected, WAL will continue to accumulate on the primary until either the standby reconnects or the replication slot is dropped.

To enable repmgr to use replication slots, set the boolean parameter use_replication_slots in repmgr.conf:

       use_replication_slots=true

Replication slots must be enabled in postgresql.conf by setting the parameter max_replication_slots to at least the number of expected standbys (changes to this parameter require a server restart).

When cloning a standby, repmgr will automatically generate an appropriate slot name, which is stored in the repmgr.nodes table, and create the slot on the upstream node:

    repmgr=# SELECT node_id, upstream_node_id, active, node_name, type, priority, slot_name
               FROM repmgr.nodes ORDER BY node_id;
     node_id | upstream_node_id | active | node_name |  type   | priority |   slot_name
    ---------+------------------+--------+-----------+---------+----------+---------------
           1 |                  | t      | node1     | primary |      100 | repmgr_slot_1
           2 |                1 | t      | node2     | standby |      100 | repmgr_slot_2
           3 |                1 | t      | node3     | standby |      100 | repmgr_slot_3
     (3 rows)

    repmgr=# SELECT slot_name, slot_type, active, active_pid FROM pg_replication_slots ;
       slot_name   | slot_type | active | active_pid
    ---------------+-----------+--------+------------
     repmgr_slot_2 | physical  | t      |      23658
     repmgr_slot_3 | physical  | t      |      23687
    (2 rows)

Note that a slot name will be created by default for the primary but not actually used unless the primary is converted to a standby using e.g. repmgr standby switchover.

Further information on replication slots in the PostgreSQL documentation: https://www.postgresql.org/docs/current/interactive/warm-standby.html#STREAMING-REPLICATION-SLOTS