What I have gathered is that it highlights useful objects or opportunities in the environment. What I cannot tell is whether that is a hold to activate ability, a passive thing, or a mission specific tool.
Also does it interact with the greet and taunt options, or are those separate systems that people keep merging?
Accepted answer
Focus is described as highlighting useful objects or opportunities. Whether it is held, toggled or contextual has not been shown, and the three descriptions you have read are three people guessing at that gap.
Greet and taunt are separate. Those are the NPC interaction options that appear when your weapon is holstered, with defuse and provoke appearing in tense situations instead.
Separate systems then. That is the bit everyone is merging and it is why the descriptions all differ.
the eavesdropping one too. stand near a conversation too long and someone notices
It also implies NPCs have a state rather than a script, since the option set changes with their state rather than with location.
defuse and provoke only showing up when things are already tense is such a good detail. the game knows what kind of moment youre in
npcs confronting you for following them too long is the detail that got me. theyre paying attention back