CLX is Cascade Lake

there are two separate lines of 5820 with different chipset.

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/us/Precision-5820-Tower-Spec-Sheet.pdf

Intel® C422 (Skylake/Cascade Lake W) ;
Intel X299 (Skylake/Cascade Lake X)

'DPC' stands for DIMM per channel

See NOTE:Here are abbreviations of the terms used in the 'Memory Matrix'-

https://www.dell.com/support/manuals/en-us/oth-xlt5820/Precision_5820_OM_pub/memory-configuration?guid=guid-ce1561c8-88a9-45e3-bc3b-762cbb37e073&lang=en-us