What does the capacity rail in the margin actually show?

It draws the next fifty-two weeks, one tick per week, running down the right-hand margin of every page. The length of each tick is the share of the people currently on screen who are still committed in that week. A long tick means nobody is free. A short stub means everybody is.

The single red tick is the crossing — the first week the group you are looking at opens up. On a profile page the rail narrows to one person and shows that person's year.

Filter the roster by discipline and the rail redraws. That is deliberate: picking a discipline is a question about capacity, not a request to shorten a list.