To get the pipework out of a room where it is on show. A surface stop valve on a chrome tail below a basin is functional and visually noisy, and behind a close-coupled pan it is awkward to reach.

Recessing puts the pipe in the wall and leaves only a face plate, which suits a bathroom where the sanitaryware is the intended focus.

It also protects the valve from being knocked by cleaning equipment, which is a real cause of weeping valves in public washrooms.

Through the face plate, which is the whole point of the product. The plate carries an opening or a removable cover aligned with the valve spindle.

A recessed valve with no access is not a supply box; it is a buried joint, and burying a serviceable valve in a wall is exactly what plumbing practice tells you not to do.

When specifying, it is worth confirming that the valve can be operated by hand through the plate rather than needing a tool that will not be to hand in an emergency.

In what it has to look like, more than in what it does. A laundry box lives behind an appliance and is rarely seen, so it is usually plain plastic with a functional cover.

A lavatory box is in the finished room, so face plates are offered in chrome and brushed finishes to sit with the tapware.

The laundry box also usually includes a drain connection, which a toilet or basin box does not, because there is no appliance discharging into it.

Dual boxes exist and are common for a basin, carrying two valves behind a single plate so both supplies are isolated in one place.

For a toilet a single cold valve is normally all that is required, so single boxes are the usual fit there.

Where a dual box is used, labelling or colour-coding the valves matters, because once recessed there is no visible pipe run to tell an installer which is which.

Enough for the housing plus the valve body, which is why these boxes are designed around standard stud depths.

In a solid wall the depth has to be chased out, and in a thin partition or a duct panel there may simply not be room for a standard housing.

Shallow-profile versions exist for exactly that case, and checking available depth before ordering avoids discovering the problem with the wall already open.

Often not, because the void serving a wall-hung pan is occupied by the prewall frame and its cistern, leaving nowhere for a separate recessed box.

Those systems normally provide their own service access through the flush plate aperture, which is where the isolation for the cistern lives.

Fitting a supply box into a duct that also carries a frame needs coordinating at first fix, and is usually placed to one side of the frame rather than behind the pan.

Commonly white, chrome and brushed stainless, chosen to sit alongside the tapware rather than to disappear into the wall.

Matt black and coloured finishes follow bathroom fashion and are increasingly offered, with the same caveat as any coated finish about cleaning products.

Because the plate is small and close to the sanitaryware, a near-match in finish tends to read worse than a deliberate contrast, which is worth considering when the exact tapware finish is unavailable.