Just in case anyone tries to find out if the great Lenovo Workstation P620 supports 1TB of RAM. I got you covered. Yes it does!
The documentation of Lenovo says different things. The Maintenance Manual talks about a maximum of 512GB. The marketing tech specs are allowing 1TB. I found proof for support in the BIOS-Update history.
To achieve this you need 8 x 128GB RDIMMs. But normally 128GB-RDIMMs are LRDIMMs. The CPU supports LRDIMMs but not the mainboard of the P620.
So you have to find the unicorn. And that is from Samsung Its correct product number is: M393AAG40M32-CAE. Thats a 128GB DDR4-3200 Registered DIMM with ECC.
Maybe this article save someone a little headache because it is not the cheapest RAM available.