This was detected / is a followup of MDL-64383.

In that issue, it was detected that some of the "required" fields (basically username, firstname, lastname and email) in the user creation/edit forms are showing different texts when the fields have not been filled:

Some of them show the "Required" text ('required' lang string), while others are showing a specialized "Missing XXXX" text ('missingxxxx' lang strings). And, curiously, the fields using "required" (username and email) do have both the 'missingusername' and 'missingemail' strings defined in the lang file.

So, this issue is about to decide which one of the alternatives to use and normalize its use over the 2 edit user screens/forms (normal and advanced).

Ciao