User engagement asks whether the product draws users in and keeps them willing to use it. A measure relates engagement behaviour to the population and period observed. Engagement is the family most easily measured badly. A count of sessions is not engagement unless it is related to the population and the task. The measure should say what counts as engaged: a completed task, a return within a period, a sustained use over time. It should also say what the product is for, because a tool used once a quarter can be highly engaging and a daily app can be a chore. The draft places it under interaction capability, so the measure is about the interaction, not about marketing reach.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.