There are these Chapters in the documentation:

• Chapter 1. Apache Tomcat Connector
• Section 1.2. Configure Load Balancing Using Apache HTTP Server and mod_jk
• Chapter 2. JBoss HTTP Connector (mod_cluster)

I find the structure somewhat confusing.

• there is Apache Tomcat; it acts as a worker node
• there is Apache HTTP Server; it acts as a balancer and proxy
• there is mod_jk Apache HTTP Server module which enables load balancing
• there is mod_cluster Apache HTTP Server module and Tomcat listener (service) which enables load balancing on the Apache HTTP Server side and on-site worker load calculation

It would be much better to restructure the aforementioned Sections so as to reflect the fact that mod_cluster and mod_jk serve the same purpose; with mod_cluster being much more powerful and feature rich, forming a superset of mod_jk's features.

Furthermore, JBoss HTTP Connector (mod_cluster) is a weird, old name that has been abandoned in EAP documentation years ago. mod_cluster is commonly referred to as mod_cluster connector these days.