Units in Guard Mode are skipped and not selected when using the two cycle unit select keybinds.
Per the image, a white "G" character is noted on the unit's counter when in Guard Mode.
Keybinds
0 - Toggle selected unit Guard Mode
Q - Clear Guard Mode from all units
Related to keybinds:
8 - Select Previous Unit
9 - Select Next Unit
See Anatomy of a Unit Counter for further details about unit counters.

Guard Mode is only mentioned in Manual section 3.3.1, Hotkeys and without a description.