Please provide your thoughts for better understanding on below senario.
VMDK base disk with RDM attached, the pointer VDMK reports the same size in the ISCSI data store as the RDM on the storage Array.
As the utilization on the RDM grows, does the RDM pointer in the iscsi data store grow with it? is it 100 GB allocated as an RDM mean 100 GB consumed in the ISCSI data store the pointer is in?
I assume you created a thin LUN on your iSCSI storage and attached it to your VM as a pRDM.
While a thin LUN doesn't necessarily consume/allocate all of it's storage the VM (or any physical host) wouldn't really care if it's a thin or thick LUN.
So either the partition table and the used file system will be created by assuming all tracks could be used.
In other words, the pointer file for a thin and a thick LUN doesn't really differ, both would contain the information about the current size of the disk (number of 512 bytes blocks/tracks).
But when you install/use the VMware tools within your VM they could report the disk utilization back to the vCenter.
That's why your vCenter could report how much capacity is used by the guest OS on the vmdk or (p)RDM.
In short, whenever the VMware tools are installed vCenter would be able to report the disk utilization, but that's not somehow related to the vmdk pointer file.