The dust cover seals the ejection port (which allows spent brass to exit the upper receiver after firing) from allowing contaminants such as sand, dirt, or other debris from entering the mechanism.
Pistols - The dust cover is used to refer to the forward part of the frame. This is where a Picatinny Rail accessory system could be found, if present.
Some people choose not to use the term Dust Cover in reference to pistols. Note that the User Manual for a Ruger P345 uses the term 'frame' for reference.[1]
Rifles -
Part number 4
Dust cover open, showing bolt
The dust cover, for example, of an M16 series rifle. This is also called an Ejection Port Cover.[1]
