The bootonly installation media does not hold copies of the installation files. When a bootonly installation method is used, the files must be retrieved over a network connection as they are needed.
After the network connection has been configured as shown in Section 3.9.2, a mirror site is selected. Mirror sites cache copies of the FreeBSD files. Choose a mirror site located in the same region of the world as the computer on which FreeBSD is being installed. Files can be retrieved more quickly when the mirror is close to the target computer, and installation time will be reduced.
Installation will continue as if the installation files were located on local media.