DDR4, 3600 MHz - PC4-28800U vs NON U and what the U means?

[Miltos Ser]

28800U VS 28800: made a quick research and i didn't find many details on this. What the U means at the end of the numbers?
I read somewhere that it means Unbuffered. But i dont think this is correct. The U cost more over the non U. Unbuffered memory is mostly use for home use and buffered to home systems. Thats why the buffered ram cost less.
Check the image. The 2 sticks cost less VS the 2 sticks of the U version.