Why does a motherboard like a SuperMicro MBD-X8DTH-6F-O support 24gb of unbuffered memory but also support 192gb of registered memory?
Chopper3 noted that registered memory allows one to install more DIMMs per memory channel. The other half of the equation is the amount of memory per DIMM. Supermicro (the mobo vendor) tested 16 GB registered DIMMs, and 2 GB unbuffered DIMMs. That isn't to say that larger unbuffered DIMMs won't work, only that supermicro didn't bother testing larger ones.
But, the final aspect is that one shouldn't discount documentation changes. Per the supermicro website (http://www.supermicro.com/products/motherboard/QPI/5500/X8DTH-6F.cfm), the UDIMM max is 48 GB (4GB DIMMs).