Someone sits under that diffuser all day and they are cold. You have moved the desk, you have asked facilities, and the air still comes straight down. The solid Elima-Draft cover closes that outlet off.
It is a 24 by 24 inch polystyrene cover, 2.75 inches deep, that holds on magnets against a compatible steel grid or diffuser face. No tools, no fixings, no electrician. It comes off the same way it went on.
This is the only configuration in the family with no opening. The other five redirect the air to one, two, three or four sides, or throw it at 45 degrees. If you want the supply air to carry on but go somewhere else, one of those is the better buy.

What does a solid diffuser cover actually do?

It stops the air coming out of that one outlet. The cover sits over the diffuser face and the discharge path is closed, so the supply air that used to drop into that spot stays in the duct and leaves through the other outlets on the run. People reach for this when the problem is not where the air goes but that there is any air at all over one desk.

When is solid the right choice, and when is it not?

Solid suits the single stubborn outlet. One diffuser, one cold complaint, a large room with plenty of other supply. It is the wrong choice if that diffuser is doing real work, because closing it pushes its share of the air to neighbouring outlets and you may just move the complaint. If the air only needs to stop hitting one person, the 3-way closes a single side and leaves the rest open.

Will it fit your diffuser?

Two measurements decide it. The diffuser needs to be a nominal 24-inch square, which is the standard ceiling grid module in most commercial buildings. And you need 2.75 inches of clearance below the ceiling line for the depth of the cover. The third check is not a measurement: the magnets need steel. Plenty of grids and diffuser faces are aluminium or painted plastic, and a magnet will not hold on either. Put a fridge magnet against it before you order.

How does it install?

You lift it up and let the magnets take it. That is the whole procedure. There is nothing to screw, clip or wire, and nothing is left behind when you take it off. Check the manufacturer's guidance for orientation, and work from a proper platform. A 24-inch square is awkward overhead even at this weight.

Does covering a diffuser affect the rest of the system?

It can, and anyone telling you otherwise is guessing. Air that cannot leave through one outlet leaves through the others, which raises flow at those positions. On one diffuser in a big open floor, nobody notices. Close several on the same branch and you may find a new draft somewhere else, or a fan working against more resistance than it was balanced for. One cover is a comfort fix. A dozen is a design change, and that deserves a word with your HVAC contractor.

Solid or one of the directional covers?

Ask what you want the air to do. If the answer is nothing at this outlet, solid. If the answer is carry on, just not at that person, you want a directional cover, and which one depends on where there is space for the air to go. One clear side, the 1-way. Two, the 2-way. One problem side and three fine ones, the 3-way. A jet that needs softening, the 4-way. A grid running diagonally to the room, the 45-degree.

Is this a filter?

No, and it is worth being plain about it because the two products look similar on a shelf. This cover changes where air goes. It has no filter media, no MERV rating and no filtration claim of any kind. If you are trying to clean the air rather than move it, you want a filtration cover, which is a different product with a different job and a different price.