Attendance tracking settings is the fourth section of an opportunity's Schedule settings. It decides how attendance gets recorded: whether people check themselves in, how they do it, whether they have to be on site, and when the check-in and check-out windows are open.
Two settings here are worth separating in your mind. Settings 1 to 4 decide how someone checks in. Settings 5 and 6 decide when they are allowed to.

While creating an opportunity, on the Customize step under Schedule. After it is published, go to Opportunities > open the opportunity > Schedule tab, then open the Attendance tracking settings card.
By default, users can check in and out by scanning the Attendance QR code in the mobile app, or anyone can check in and out at an Attendance Kiosk. Those two routes work without touching any of the settings below. Everything in this section either adds another route, restricts one, or moves the window.
Controls whether the check-in and check-out functions exist at all for this opportunity's shifts. They are two separate switches:
Allow check-in. On by default.
Allow check-out. On by default.
Turning check-out off is the common case. If you already know a shift's length and do not need people to sign out, switch it off and the hours are recorded without a second action.
Adds a button on the shift in the mobile app, so someone can confirm attendance with one tap instead of finding and scanning the Attendance QR code.
Enable it for programs where handing out a printed code is impractical, for example people working alone or off site. Leave it off where the scan is the point, because the QR code is what ties the check-in to a physical location.
Requires the user to be within a radius you set, in metres, of the shift location before they can check in or out.
This is the counterweight to tap-to-check-in. Together they give you a one-tap check-in that can still only happen on site.
Checks attendees in and out automatically around the scheduled shift times, with no action from anyone. A shift starting at 2:00 PM auto checks in every attendee at 2:00 PM, and a 4:00 PM end time auto checks them out at 4:00 PM.
Auto check-in and auto check-out are separate switches, and each takes an offset in minutes so you can run the automation earlier or later than the scheduled time.

By default the check-in portal opens 1 hour before the shift start time. Three options:
Opens 1 hour before the shift start time. The default.
Available at any time on the day of a shift. Good for a day-long event where people drift in.
Set a custom check-in window, in hours and minutes before the start time.
By default the check-out portal closes 1 hour after the shift end time. The same three shapes:
Closes 1 hour after the shift end time. The default.
Available at any time on the day of a shift.
Set a custom check-out window, in hours and minutes after the end time.
Widen this window if you regularly get people forgetting to sign out and having to ask you to fix their hours.
Out of the box, the Attendance QR code and the Attendance Kiosk both work with no configuration.
Check-in and check-out are separate switches, so you can record arrival only.
Tap-to-check-in removes the scan. Proximity validation puts the location requirement back.
Auto check-in/out logs attendance from the schedule rather than from a real arrival, and takes minute offsets on each side.
The portal windows default to 1 hour before the start and 1 hour after the end, and both can be opened for the whole day or set to a custom window.
Administrators can always record or correct attendance regardless of these settings.