Focus is in badminton the addressing of attention.

The subject of the focus is often not mentioned further. When playing badminton, one has to focus, and that's that. But often is meant "the game", and that attention should being directed on it.

Attention and focus
So, in fact, there are two objects, attention and focus.
Attention is the being aware.
Focus is the being aware of a subject.

Attention doesn't need necessarily a specific subject. Just being there and being aware is enough. Eyes, ears, nose do the rest. Attention isn't directed at anything then. Things are just coming through as they come. It's the way of mindfullness, which is, paradoxically, making the head empty.
When playing badminton a choice is made. Things are still coming through as they come, but now things are the badminton game allone. So, attention is directed at the game now. A focus is on it. Being aware is directed at a subject.

When being aware of a subject, so focusing, things can still come through as they are. The subject then is everything about the match, but in fact just of the performance.

When looking at a match, attention can directed further. For example just at who will win (and with that looking on then being involved and experiencing the tension of the match). But also on many other subjects.
Focused can be on placing the shuttle, positioning of the players or of one player, the flights of the shuttle to the various places, movements over the court of players or of one player, techniques, one particular stroke, footwork of one player, the behaviour of a player, the rapidity of playing, etcetera.

When playing self, attention is needed for the game. So, attention has to be focused on it.
When playing, in fact, focus is on achieving unreturnable placing of the shuttle. That delivers points and with that the win of the match.
When focussing on achieving unreturnable shots, not only that ones that gain points is the subject of attention of course. Gaining points is established by deliberately placing the shuttle in rallies. So, the focus is going on.

When playing, focused is on gaining points. That should be clarified.
When gaining points, the whole game should be seen. That means, the court with its lines and net, the opponent or opponents and their places and movements on court, and the partner and one self and the places and movements on court, and of course the shuttle and its flights.
When perceiving this, it can be acknowledged how positions can be taken and the shuttle can be placed.
And with that in mind, a tactical plan should be played.

How to establish such a whole-play focus?
Its done by emptying the head and just perceving what occurs and happens in the game. One should be aware constantly of the whole game.
Concretely, when looking at the shuttle, the rest is being aware of, and also really being seen from the corners of the eyes. When looking at the opponents side, perceived is nevertheless where the partner is and one self. Also perceived are the own body-movements and strokes.
So, one is aware of one self, consciously playing in that environment of the game.
And so is more extensively explained the subject of the focus.
By the way, during such a game focus might slightly shift to some temporarily asking occurency as a shuttle or the partner and such.

On focusing
Here above is focus described as a relaxing and open way of directing attention. Often focus is described also as forcing attention to the game. The choice of doing might be personally. In these pages playing with an open mind and a relaxed perception is seen as most advantageous for playing, learning and pleasure.

Focus on technique and tactic
Often are techniques and tactics not given only as a possibly good way for playing, but as being the way that it is done, so as a kind of rule. In that case a further focus takes place by looking and playing by that way. An example is the doubles systems people sometimes follow to play. In these pages, techniques and tactics are left open, which means, these should be developed after experiences and trying out things. But, of course, the many good examples are very useful.