What is Xeon D's use case? Middle-cost, compared to Goldmont below, and Skylake-SP and W above? Not sure: they exist, but not a lot of discussion about it.
Sorry to attempt an answer at this late, but it looked like everyone ignored your inquiry.
Intel seems to have positioned Xeon D as a low-power server/workstation blade CPU. It appears to be overall the most efficient lineup of Xeon processors available. Or at least it was when it launched, and it remained so when Intel started showing off Denverton as well. Not really sure how Broadwell-based Xeon-D fares against Skylake-based Xeon in terms of raw efficiency.
The downside to Xeon D is that it has to remain within a fairly low clockspeed envelope - generally below 3 GHz - to meet power targets. So it's not well-suited to applications that require high individual core performance (low-latency VMs, etc).
Near as I can tell, Intel's general "professional" CPU stack goes a bit like this:
Denverton - network appliances and suchlike
Xeon-D - low-power perf/watt applications, preferably those that are highly-scalable with core/thread count.
Xeon - everything else