NBD and NBDSSL

Folks,

When I use VDDK(1.2.1) to write to the VMDK files of a VM, NBDSSL was used for some disks(VMDKs) and NBD was used for some disks of the same VM. Any idea for this? Any specific configutaion which can force NBDSSL over NBD?

Thanks in Advance,

-Mani.