vApplyHD is having a vApplyHD module on every single row whereas vApply FlowSense is having a vApplyHD module control a divisible number of rows into sections based on how they're plumbed and programmed into the Gen 2 or 3 20/20.

I would not recommend doing vApply FlowSense if I were considering it for my own operation.  Works well and has terrific visibilty compared to a Redball system or flowmeter after the pump only, BUT getting the planter plumbed for even distribution to the fertilizer attachments leaves the door open for lots of row to row variabilty and the density of different products can prove to be a real headache when trying to reach your target rates in the field.  Pump calibration (if using a controlled pump style system) will pass with water in your farm yard where you can check it, but then switching to fertilizer proves to be a whole different set of challenges for the system.  That's for another thread on NAT though...

vApplyHD is expensive but its the absolute "cat's meow" when concerning ease of installation, as-applied accuracy, and is self-compensating around curves, challenging prescription maps, and other undulating factors that may come into play.  Well worth the money if you do any kind of variable rate fertilizer program and use the planter pass as part of nutrient application or if you get the vApplyHD Flex modules for use on a planter and a sidedress bar.