Registration is done automatically by for each master descriptive file that contains the d and R flags. Autoregistration is done at bootstrap phase 2. It is initiated by the script /etc/rc2/S23autoconfig. Registration can be done manually at any time after bootstrap by using the ml or lboot command with the reg option (see the ml(1M) and lboot(1M) reference pages).
Once it has been registered, a driver is loaded automatically the first time a process attempts to open a device special file with the major device number of this driver. You can also load a registered driver in advance of any use with the ml or lboot command--loading implies registration.