The AFRINIC WHOIS database has a daily query limit that is applicable to the Person Objects only. Each IP address querying the database has a default daily limit of 5000 queries the limit is reached within 24 hours, the IP address is temporarily blocked: an event of type "BLOCK_TEMPORARY" is inserted in the DB (acl_event). The IP address will be unblocked after 24 hours max (maybe less, depending on the schedule that runs every 24 hours).
Note that the "BLOCK_TEMPORARY" event is not deleted from the DB. It will be deleted after 3 months. If the same IP address was blocked more than 10 times in the last 3 months, then it becomes blocked permanently. An IP address that was blocked permanently won't be able to query the WHOIS for one year.
Some users may have to do a lot of queries legitimately. To avoid these IPs to be blocked, a special limit can be set in the DB. It is the purpose of the acl_limit table in the ACL_LOCAL database