A shared disk exists for the times where your organisation might require two or more virtual machines to access a common set of data on an individual disk. While in theory it could certainly be possibly to mount a regular disk across two systems, a typical file systems' attempt to read/write a file accessible by another machine could have undesired consequences; namely, data corruption. Shared storage systems coupled with filesystems designed for said purpose will coordinate access to a file and ensure that read/write actions are handled in a safe manner.
A traditional system disk on a given virtual machine can only be mounted to the machine it relates to; whereas, the Katapult Shared Disks functionality affords the ability to create a single disk which can then be assigned to one or more of your virtual machines. Once a shared disk is assigned to a virtual machine you can then machine the disk, i.e. mount/detach, from within the virtual machine in the Katapult Console.
As mentioned in the what is a shared disk? section above, a shared disk in Katapult is no different to your regular system disk apart from the fact it can be mounted across machines. Thus, from the management perspective; within the console at least, once a shared disk has been created the interface is largely universal between the two disk types. For example, clicking into the overview screen for a system disk (via a virtual machine Disks -> System disk menu) or shared disk (via _Storage -> Shared disks sidebar menu) affords the same disk management functionality.
Katapult shared disks will support any cluster-aware file systems, such as OCFS2. Other options are available and we do not restrict which option you choose.