A formal method for designing and representing human-computer interaction dialogues using box and line diagrams is called:.

Dialogue Diagramming

Explanation:

Dialogue is simply a sequence of interaction that usually occurs between a user and a system.it consists of Designing the dialogue sequence, Building a prototype and Assessing Usability

Dialogue diagramming is a known widely anf formal means by which we are designing and representing human-computer dialogues using box & line diagrams.