A window is a promise made by the office and paid for by the route.
Why it matters
Windows are a constraint in the mathematical sense: each one removes possible orders in which the day can be driven. One two-hour window on a twelve-stop route is almost free. Five of them, scattered across the map, can make a route that cannot be driven efficiently at all — not because the planner is weak, but because the day was over-promised before planning began.
How you notice it slipping
Windows get narrower over time without anyone deciding to narrow them. A two-hour window becomes ninety minutes because a customer asked and it seemed harmless. The share of appointments carrying a window rises, often because it has become the default answer rather than a choice.
What to do
Treat the window as inventory. Count what share of appointments carry one and how tight they are, by booking channel and by person. Then give the booking team a ladder: a service day first, a half-day if the customer needs it, a narrow window only when there is a real constraint on the other side. Most customers accept the widest answer they are offered.