The understood subject in a request or command which is not usually stated?

The unstated subject of a sentence of command is most often you, the person spoken to.

The subject of this type of sentence is called the implied subject.

Examples:

Stop! (You stop!)

Put the baby in the playpen. (You put the baby in the playpen.)

Go do your homework. (You go do your homework.)

Examples of an implied subjects that are not directly the person spoken to:

Help! (Someone help!)

Run! Run for your lives! (Everyone Run!)