A specified or unspecified number of pieces must be added, so that the diagram becomes legal or a goal (e.g. #2) becomes achievable.
Sometimes the placement of the pieces can be determined independently of the goal. The #2 or whatever then a sort of secondary problem.
It would be incorrect generally to read such a problem as meaning e.g. "Add pieces so that #2 is without cooks."
An Illegal Cluster is another, very specific type of problem where pieces are added to the board. Those problems are generally not given the keyword "Add Pieces".