Ceiling grids run square to the building. Rooms do not always agree. When the desks sit on a diagonal, when a partition cuts across the grid at an angle, or when the space the air needs to reach is off the corner of the diffuser rather than off its face, the 45-degree is the configuration that lines up.
It throws the discharge at an angle to the grid instead of square off the sides. That is a different thing from a four-way, which sends air off all four faces, and the two are easy to confuse on a purchase order.
Standard body: 24 by 24 inches, 2.75 inches deep, polystyrene, held on magnets against a compatible steel grid or diffuser face.

What does a 45-degree diffuser cover do?

It angles the discharge relative to the ceiling grid rather than sending it square off the sides. Every other configuration in this family works on the four faces of the diffuser: open one, two, three or all of them. The 45-degree changes the reference instead, throwing the air at a diagonal. When the geometry of the room does not match the geometry of the grid, that is the difference that matters.

How is 45-degree different from 4-way?

They are not the same part and should not be treated as interchangeable. The 4-way spreads air off all four faces of the diffuser, square to the grid. The 45-degree throws at an angle to it. The model numbers differ, the airflow pattern differs, and because the names sound similar this is the most common ordering error in the family. If what you want is a wide soft spread, you want the 4-way. If you want the air pointed at a diagonal, you want this one.

When does an angled throw actually help?

When the furniture and the ceiling disagree. Desks laid out on a diagonal across an open floor. A room where the usable space runs corner to corner rather than wall to wall. A partition or a run of shelving crossing the grid at an angle, where a square throw would hit it flat and bounce back. In each case a square discharge sends air into an obstruction or into a dead corner, while an angled one follows the line the room actually takes. Stand in the space and look at where people and furniture sit rather than at the ceiling tiles, because the tiles are what misled whoever specified the diffuser in the first place.

Will it fit your diffuser?

The same three checks as every cover in this family. A nominal 24-inch-square diffuser, which is the standard ceiling grid module. At least 2.75 inches of clearance below the ceiling line. And a steel attachment surface, because the cover holds on magnets alone. Aluminium grids and painted plastic faces will not work, and a fridge magnet answers that in seconds.

How does it install?

Lift it, let the magnets take it, with the angle pointing where you want the air to go. Orientation carries the whole function here, so take a moment to sight along the direction before you commit. Then fit it, stand in the affected area and feel the result. If the angle is wrong it lifts straight off, and a quarter turn is the difference between right and useless.

Does this affect the rest of the system?

About as much as any other directional cover, which is to say not much at a single outlet. The diffuser passes the same volume, it simply leaves on a different line. Where angled throws can surprise you is in the interaction with other outlets nearby, because the air travels along a path the designer did not plan for. On one diffuser that is a comfort fix. Across several, watch for a new draft where two streams meet.

Is this a filter?

No. It redirects air and does nothing to its quality. No filter media, no MERV rating, no filtration claim. Filtration is a separate product with a separate purpose.