Welcome to the VMware Communities forums. The flags you've mentioned can create either virtual (createrdm) or physical (createrdmpassthru) RDM. With a virtual RDM you can take snapshots of the VM which is not the case with a physical RDM. With a physical RDM you can utilize SAN management software within the VM and it is a requirement to use physical RDMs if you plan to cluster a virtual machine with a physical host.
With ESXi, physical RDMs can exceed 2 TB, virtual RDMS can not.
Some more reading on the topic here - http://www.virtuallifestyle.nl/2010/01/recommended-detailed-material-on-rdms/
