IBM HTTP Server regulates the maximum number of concurrent processing threads in the mpm worker with the MaxClients configuration. These may be distributed across one or more processes with the ServerLimit and ThreadsPerChild configurations. MaxClients must be <= (ServerLimit * ThreadsPerChild).
If MaxClients is < (ServerLimit * ThreadsPerChild), then the governing value is MaxClients. The default configuration in IHS <= 8.5 is MaxClients 600, ServerLimit 64 and ThreadsPerChild 25, so ServerLimit * ThreadsPerChild is 1600. Why might you want this? Because this gives you the flexibility to dynamically increase MaxClients up to the ceiling of ServerLimit * ThreadsPerChild and gracefully restart IHS without destroying existing connections or waiting for them to drain. This is a useful capability but few customers take advantage of it.
You may want to simply set MaxClients = ServerLimit * ThreadsPerChild (where MaxClients is determined based on your needs and available resources) to avoid any confusion.