Many of us follow a set of steps when installing a machine. All freshly installed hosts of a specific operating system revision have a common SSH server configuration. My machines all have
tcsh installed and attach to the central authentication system. You probably have your own list. These tasks can be done by hand after installation, but it’s much easier to let OpenBSD do them for you during the installation process.
Installations can be customized by adding files during installation or by running commands after the installation.
A custom file set includes files that you want copied to your new installation. I use custom file sets to install the default /etc/sudoers, a SSH server configuration, ...