Rambus Memory is commonly referred to as a RIMM. It doesn't stand for anything in particular other than Rambus the company wanting to stay in line with the other terms like SIMM and DIMM.
RIMM is the trademarked name for a Direct Rambus memory module. RIMMs look similar to DIMMs, but have a different pin count. RIMMs transfer data in 16-bit chunks. The faster access and transfer speed generates more heat. An aluminum sheath, called a heat spreader, covers the module to protect the chips from overheating.
A CRIMM, which is what you are referring to in you in the extra slots of your system in a Continuity RIMM. With an earlier Rambus system, you must install the modules in pairs and all slots must be populated by something to keep the signal going. Thus, this is why there are CRIMMs in the remaining slots. Basically it is a PCB without the chips. You can pull out the CRIMMs when you are looking to upgrade to more memory. As long as you install the memory in pairs and have all the slots occupied.
Check out Kingston site at http://www.kingston.com
for more information on Rambus technology and other memory technologies.