Registered ECC (RDIMM) is what you need.

Your Xeon E5 v3 board expects registered DDR4 ECC, not unbuffered. Unbuffered sticks (UDIMM) are rare for this platform and often won’t work — or will run at reduced performance.

Most LGA 2011-3 boards from that era (Supermicro, ASRock, Gigabyte workstation boards) are designed for RDIMMs. The memory controller on the E5 v3 is built for registered memory’s extra buffering, especially with multiple ranks and higher capacities. UDIMMs might work in a pinch on some consumer boards like the ASUS X99 series if you stick to 2 slots, but it’s not guaranteed.

Check your motherboard’s QVL if you want to be safe. But generally, just buy PC4-2133 or 2400 registered ECC sticks off eBay. They’re cheap and plentiful.

Don’t overthink it — registered is the right call here.