Chassis differences and what to buy

Hello, how are you? i have a question for you about ps4 chassis.

I'm planning on buying a ps4 (not pro) around october, and in some stores i found some that said (Chassis-D), i got curious so i google it, i don't find out many information about the d but a some for the a, b, c... So my question is What is this Chassis D? what are the improvements, what do you recomend me to buy, also what are the number after the CUH-? how do i recognize the xhassis to buy? also buy online or in store? Thank you.