What does the /u flag do?

Acrobat DC registers itself as a custom URL protocol handler with the /u flags, e.g.

"C:\Program Files (x86)\Adobe\Acrobat DC\Acrobat\Acrobat.exe" /u "%1"

What does the /u flag do? As far as I can tell, without the flag the launcher works, but with the flag it never opens the file you provide