In a situation, where the two units are starting beside each other like this:
a = attacking unit
d = defending unit
c = centerpoint
- = Card
The attacking unit is on Close.
Will the attaking unit move the front center point to the flank center point of the defender and charge?
Or will the attacking unit attack with his flank?
Or can the activ player chose?
The attacking unit will attack on the flank.
D is clearly visible, so it is eligible for final rush.
The flank is the 'facing side'.
Nothing is preventing an engagement of front to flank.
D's flank is within final rush range.
That's all you need.
I think he was asking whether unit A would turn 90 degrees to the left and attack unit D's flank with it's front or would unit A move a little bit forward, then final rush unit D with it's own flank.
The rules for final rushing with your flank are probably the most confusing rules in the whole rulebook. A few years ago when I started playing, I asked a whole series of clarifying questions. What it basically boils down to is this: the only time you'll ever final rush using your flank instead of your front is if there's not enough physical room for you to fit the card in if you rush with your front, but there is enough room if you rush with your flank. (This can happen due to the presence of other units, or terrain, or the table edge.)
I am not aware of any other conditions that allow you to final rush with your flank.