A block snapshot maps a cloud snapshot. It can be shared with anyone on RosettaHub.
A cloud snapshot can be mapped by more than one RosettaHub block snapshot allowing for a fine grained access control. To use a cloud snapshot, you need to create a block storage from the snapshot and use that block storage as a working volume or reference volume of a RosettaHUB formation. You can easily share a block snapshot on RosettaHub, any user with access to your block snapshot will be able to create a block storage from that snapshot on their own cloud account and use it in RosettaHub formations.