The person knocking softly may be respecting rest or hesitating about their right to interrupt.

A quiet knock carries two possible meanings and gives you no way to tell them apart from the sound alone. One is care: someone protecting your sleep or your concentration. The other is doubt: someone unsure whether they are allowed to want your attention at all. Households and workplaces are full of gestures like this, small and ambiguous, where the same movement can come from consideration or from feeling slightly unwelcome.

Where you have some authority in a room, say plainly when interruption is fine and when it is not. Guessing is tiring for the person outside the door, and one clear sentence saves them a week of it.