Ghadiyadhruvik,  The memory population rules will be specific to each server models, so you would need to be more specific on which server you need it for. Now as far as the differences in the dimms.  RDIMMS - RDIMMs add a register, which buffers the address and command signals.The integrated memory controller in the CPU sees the register instead of addressing the memory chips directly. UDIMMS - The memory controller inside your CPU addresses each memory chip of your UDIMM individually and in parallel. However, each memory chip places a certain amount of capacitance on the memory channel and thus weakens the high frequency signals through that channel. As a result, the channel can only take a limited number of memory chips. LRDIMMS - Load Reduced DIMMs replace the register with an Isolation Memory Buffer component. The iMB buffers the Command, Address, and data signals. Let me know what model server you are working with and I can get the specific memory population rules for you. You can also find the memory population rules in the manual for that specific system. 
Hi Team, i have confusion in rules of memory population in dell poweredge servers for all generation.  Please provide me rules and difference between udimm, rdimm,lrdimm.  Regards, Dhruvik
memory population rules