Interesting read from laptop media but may apply to desktop too when you plan to upgrade memory.

Conclusion: The performance benefit of the 1Rx8 memory over 1Rx16 memory is so big that it is recommended not to install a 1Rx16 memory module in gaming pc.

1R refers to single rank.  The x8 or x16 refers to the bus width of the individual memory chips, and is related to the arrangement of the internal memory banks. As each rank is 64 bits, a 1Rx8 memory module will have 64/8=8 memory chips on it, and a 1Rx16 memory module will have 64/16=4 memory chips on it.
Modules using x16 chips are usually cheaper but almost always slower than those using x8, due to having fewer banks, which effectively increases latency and reduces bandwidth because accessing data in different banks is faster than accessing data in the same bank, and also due to the use of higher-density nodes with lower manufacturing costs but worse stability at low latencies and high clock speeds. (reference Quora)

For example, Dell oem 

9CXF2

DIMM,8GB,3200,1RX16,16,DDR4,NU

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