How to retrieve memory type (rdimm or udimm)

Use OCS Inventory:

Information about Hardware and Operating System are collected.

Memory slot arrays : Caption and description, Capacity in MB, Purpose (system memory, flash memory...), Type of memory (SDRAM, DDR...), Speed in MHz, Slot number.

Two reasons leap to mind but both are due to the same technology.

Basically Registered memory can wait for writes to 'fill' the available buffer, it's a bit like a synchronous link in that respect. This means that all of the reads or write collecting at the start or end of a edge signal - this makes it faster overall than unbuffered.

Also because the clock management is more accurate/adhered-to-better more modules per channel/bus can be used as the signals don't get out of sync or degrade, meaning more memory per channel/bus. That's why very often a server can take say 18 x Registered modules but only say 6 unbuffered ones. Of course unbuffered is cheaper so if you want a low-memory server it may be just fine but if you think you're likely to go over the server's 'unbuffered-limit' then you should go straight for Registered on day-one.

It's got little to do with reliability by the way - just speed and expansion.

Hope this helps.