It's not possible to have a subset of disks in a diskgroup belong to a tier - the tier will contain ALL the disks in the diskgroup. It is possible, however, to have two separate tiers defined on the same diskgroup.  Movements to the tier will potentially use all the disks, however. The only way to truly separate the disk resources is to have the disks in separate diskgroups. The diskgroups cannot be manipulated via software (Unisphere or SE). You will need to work with your local account team to come up with a plan for reconfiguring the diskgroups.  It may be possible to do this without disrupting the data if the sane RAID affinity groups are maintained when configuring the new diskgroup.  After things are reconfigured, you can use VLUN to migrate devices between diskgroups to get the right devices on the right set of disks.
I am  looking to create a series of DP tiers on a VMAX to support Raid 10 configuration for an exchange enviroment. The design I am working from calls for two DP tiers one containing 38 disk the other 8 disk to be created from a single physical disk group . However I see no way to actually specify what number of disk are in each group, it allows me to specify the disk group but not the actual number of disks etc. Is there a way to actually specify what is in each DP tier  in terms of disks  The plan  as far as I can tell is to create these two DP tiers as mirrored disk at the physical level then create stripped meta devices across these to give me raid 10.  I can just create a series of raid 1 volumes directly from the disk group to give me the same effect but would rather use the DP tier function if I can find a way to actually create more than one DP tier group from the same disk group
DP Tier Storage creation