Ranks

A forest maps a role's knowledge; ranks map the journey through it. A rank is a seniority level — junior / mid / senior by default — and each rank is defined as a concrete set of skills, at given levels, that a person at that rank is expected to hold.
The key subtlety: ranks attach to a person, not to a thing. A skill isn't "a senior skill" in nature; the organisation decides that holding it (at some level) is part of what senior means here. Setting up ranks is exactly that act: for each forest, choosing which skills — and which levels of them — belong to each rank. Different organisations will slice the same forest differently, and that's fine.
What ranks buy you:
A visible "you are here." A person reads their rank off the forest: they hold the junior set, most of the mid set, and there's the gap.
A promotion with a shape. "Getting to senior" stops being a feeling and becomes a specific, finite list of skills at levels. Both the person and their manager can see it all year, not just at review time.
A shared language for expectations. "Mid" means the same set of skills to everyone looking at the same forest — no private definitions.
On names: junior / mid / senior is the default ladder, but an alternative like apprentice / journeyman / master maps onto exactly the same structure. Pick whichever fits the culture; the mechanics don't change.
Ranks are also where the method eventually touches evaluation and pay — a powerful and delicate connection that gets its own treatment in the skill-management chapter.