See the Foreman manual for general information on configuring provisioning.
Katello ships a number of templates in addition to the standard Foreman ones. When using these templates, if a host has a Host group with an Activation Key, it will register as a Content Host automatically.
To customize any of the above templates, simply clone them and add your changes.
When you synchronize a repository with a distribution such as Fedora or CentOS, Katello will automatically create the operating system and assign these default templates. You may change the defaults by going to Administer > Settings, and selecting the Katello tab.