It is the size of the window that gets closed when that weekday's rule fires. A value of N on weekday W closes every delivery date from W itself through W + N days — the page spells the list out for you under the row (for N = 2 on Friday: Friday, Saturday, Sunday).

• 0 closes only that same day (useful for same-day cut-offs such as "orders for today close at 10:00").
• 1 closes today and tomorrow, 2 closes today plus the next two days, and so on up to 7.

Behind the scenes the scheduled job closes all open delivery dates earlier than now + N days once the clock passes the Time to close on that weekday. A delivery date that lies inside several weekdays' windows is closed by the earliest rule that reaches it.