What is Sending window?
The hours and days during which a campaign is allowed to send, usually business hours in the recipient's timezone.
Mail that arrives at 3 a.m. is read after the morning's real mail, if at all, and off-hours sending is itself a bulk-sender pattern. A sending window restricts sends to, say, 09:00–16:30 on weekdays, either in the campaign's timezone or in each recipient's own timezone if you have it. Steps that come due outside the window wait for it to open.