I wish they'd just standardized on specific numbering systems for cameras intended for a specific market segment from the very start and stuck with it.
3xxx, 5xxx, and 7xxx series have been pretty consistent, and so have the sports-oriented, high continuous shutter speed D1 thru D5 cameras.
but the 3 digit number series (D100 thru D810) are a mix of both full and crop sensor cameras, some are prosumer, others are full professional.
MAKE UP YOUR MIND, NIKON.
I would prefer that from this moment forward, the 3 digit series would be reserved for the FX, professional, max res models, the 1 digit series stays the same until they run out of numbers, then go to a 2 digit series, since the 2 digit series now is retired (D40, 50, 60, 70, 80, and 90) and keep the 3K, 5K, and 7K series right where it is.
Once you understand their numbering system, ideally you'd know exactly where a new model fit in as soon as you knew the model number.
Here's how I'd structure them if I had anything to say about it. (I don't.)
3xxx series, entry level DSLR. DX.
5xxx series, enthusiast level. Higher resolution, more bells and whistles, DX
7xxx series, prosumer level, emphasis on performance over bells and whistles, DX
9xxx, prosumer with FX format,
XXX, professional, high res, FX format
X, professional, high frame speed, sports oriented, FX format
While I'm ranting, I've got to say it: I'm of the opinion that removing AUTO and SCENE modes from the pro grade cameras (D4, D810) was a mistake. Even for an authentic, certified, much worshipped professional photographer, there are times when having an AUTO mode or a preset scene mode would be appreciated. And it's just software.