The model concentrates on how the leader behaves towards followers, which the authors treat as the decisive factor in any leadership situation, because followers individually accept or reject the leader and as a group decide what personal power the leader has. Readiness (maturity in the earlier editions, Performance Readiness in the current model) is how far a follower is able and willing to carry out a particular task. It is not a personal characteristic, trait, value or age; nobody is ready or unready in a total sense. Ability is the knowledge, experience and skill brought to the task (job readiness, from education or experience, which the authors treat as equivalent); willingness is confidence, commitment and motivation (psychological readiness, linked to achievement motivation and to setting goals that are demanding yet reachable). The two interact, so a change in one moves the other. Before readiness can be judged the leader must make the task-specific outcomes, objectives, subtasks and milestones clear. A group can have its own readiness as a group, different from that of its members, and the leader may need to behave differently one-to-one.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.