As seen in the image, there's an org with (D) and an org with (U). The (D) means default as per the Trailhead modules, but there's never any explanation as to what (U) means in any module or docs. What is it?
It stands for the default scratch org username.
A (U) on the left points to the default scratch org username.
The default Dev Hub and scratch orgs are marked on the left with (D) and (U), respectively.