Anyone know what the 'U' stands for in '10GBASE-CU'?

[klegerton]

Just as the title says, it is driving me crazy not being able to find what that letter stands for. I know these are 10G SFP+ DAC cables. Some companies write it as 10GBASE-CR which stands for:

10G = '10GbE'

BASE = 'baseband'

C = 'Copper backplain

R = '64B/66B block coding for 10G+ speeds

What the heck does the U stand for???

[Philip D'Ath · Accepted solution]

The same question was asked on reddit 1 hour before your question here.  The consensus seems to be copper, since Cu is the chemical symbol for copper.