Frontier

A frontier is the set of nodes whose prerequisites are all satisfied — the edge of what is takeable now. The word is borrowed from graph search, where the frontier is the set of generated but not-yet-expanded nodes and its defining property is that it separates the explored part of the graph from the unexplored. On a tracker that is items which are ready-labelled, unassigned, and blocked neither by a dependency edge nor by a label. In an interview it is every decision whose prerequisites are already settled. Same word, same meaning, two surfaces. An empty frontier is a fact that needs a reason attached — all claimed, triage stalled, or everything blocked — because an empty answer with no breakdown sends people guessing.