Safely - When you safely reserve a diskset, DiskSuite checks to see if another host currently has the set reserved. If another host has the diskset reserved, your host will not be allowed to reserve the set.
Forcibly - When you forcibly reserve a diskset, DiskSuite reserves the diskset whether or not another host currently has the set reserved. This method is generally used when a host in the diskset is down or not communicating. All disks within the set are taken over and FailFast is enabled. The metadevice state database is read in on the host performing the reservation and the shared metadevices configured in the set become accessible. If the other host had the diskset reserved at this point, it would panic due to reservation loss.
Normally, two hosts in a diskset cooperate with each other to ensure that drives in a diskset are reserved by only one host at a time. A normal situation is defined as both hosts up and communicating with each other.