What is the meaning of the 51/52 of NRF51/52?

[Andrew]

I am just curious, do the numbers 51 and the 52 have any meaning? Does the base of 50 imply anything? Are these just arbitrary numbers for the sake of good marketing?

[Nguyen Hoan Hoang]

51 or nRF51 refers to the nRF51822, nRF51422 series SoC (Cortex-M0) 52 or nRF52 refers to the nRF52832, nRf2840, nRF52180 series SoC (Cortex-M4F)

They represent 2 different family of Soc