Since you dug up the thread why didn't you take the time to read it? The very thing you are asking is clearly answered.
1. IF you shoot at a unit you can only assault that unit.
2. An exception is provided allowing a unit to assault the passengers of a vehicle that
that unit destroyed.
3. A unit that didn't shoot at anything can naturally assault whatever unit it likes....following the rules for Assaulting, of course.
So there are your answers.
You don't really need to see this as the answers are in the above rules