Always put subject heading so that the person receiving the email can quickly see what the email is about.
The subject in the sentence "The box is very big" is "The box."
it is the box where you input your purpose
The purpose of a Combo Box is so that a person gets more food or whatever comes in the combo box, for less of a price then if they bought everything separate.
the purpose of a quick change gear box (as the name suggests) is to allow you to change gear quickly.
Subject occasion audience purpose (style)
The purpose of an HDMI splitter box is usually to connect multiple high-definition devices to an HDTV. The purpose of an HDMI splitter box is usually to connect multiple high-definition devices to an purpose of an HDMI splitter box is usually to connect multiple high-definition devices to an HDTV?
The purpose of an electrical knockout in a junction box is to provide a way to create openings for electrical conduits or cables to enter the box, allowing for the connection of wires and cables within the box for electrical connections.
The purpose of the box wrench is to have a flat of the wrench on each flat of the nut, thus giving more turning effort. - I always use box wrenches on old, rusted or stubborn nuts.
its to reduce noise
The purpose of a sound box in a musical instrument is to amplify and project the sound produced by the instrument, making it louder and richer in tone.
The subject pronoun it will take the place of the subject noun phrase 'the lid of the box': It was left wide open.