The purpose of this success criterion is to help a person know which element has the keyboard focus.
It must be possible for a person to know which element among multiple elements has the keyboard focus. If there is only one keyboard actionable control on the screen, the success criterion would be met because the visual design presents only one keyboard actionable item.
Where the success criterion says “mode of operation”, this is to account for platforms which may not always show a focus indicator. In most cases there is only one mode of operation so this success criteria applies.
Note that a keyboard focus indicator can take different forms. This criterion does not specify what the form is, but Focus Visible (Enhanced) does define how visible the indicator should be. Passing Focus Visible (Enhanced) would pass this success criterion.