If the system fails during installation of an update or optional
software product, VMSINSTAL.COM
attempts to continue the installation upon rebooting. Depending
on when the system failed, one of three conditions exists:
The system disk was not altered before
the system failure. In this case, VMSINSTAL.COM instructs you to
restart the installation.
The system disk or a library used by the installation
was corrupted. In this case, VMSINSTAL.COM instructs you to restore
either the system disk or the corrupted library from the backup
copy and to restart the installation.
VMSINSTAL.COM continues the installation. In this
case, the procedure performs most of the installation. In addition,
VMSINSTAL.COM might tell you that you must perform some tasks manually to
complete the installation.
HP OpenVMS System Manager's Manual, Volume 1:... 
Installing, Upgrading, and Updating Software
