As Tim eluded to in an earlier post on this thread..... SUBs can receive DHCP forwarded messages, it will however just forward them to the PUB and then the PUB consolidates all this info, which can come from different profiling sources... DHCP, OUI, HTTP-Agent, TCP Fingerprinting, Cisco sensor etc. and then processes this data and uses as discussed in my below technote to come up with a fingerprint that is the most specific and the most 'trust-worthy' and then syn'c this back to the cluster.
ClearPass Profiling TechNote V1.1.pdf