The HTTWSB is a wall sleeve designed for through-the-wall (built-in) air conditioner models/series:
They come with the solid back panel that protects the opening before the air conditioner is installed (ex. pre-installed sleeves on a construction job site). It is removed and replaced by a louvered or plastic mesh screen, which should come with the sleeve or air conditioner.
This is an accessory and not a part. It is available through some online dealers.
This is not intended for window units, which have the side louvers/vents that would be blocked by the sleeve.