User reservation settings is the first section of an opportunity's Schedule settings. It decides what an approved user is allowed to do on their own: whether they can see the shifts, reserve them, cancel them, and how many they can take.
These settings apply to users only. An administrator can always assign, reserve and cancel on someone's behalf regardless of what is configured here.

Two places, and they are the same settings either way:
While creating an opportunity, on the Customize step, under Schedule.
After it is published, go to Opportunities > open the opportunity > Schedule tab.
Each row shows the option currently applied. Click anywhere on the row, or the Edit pill, to change it.
By default, once a user is approved for the opportunity they can see all of its upcoming shifts and reserve them. This setting narrows that down. Three options:
Approved users can view and reserve shifts. The default.
Approved users can view shifts, but cannot reserve them. Useful when you want people to see what is coming up but you decide who goes where.
Approved users cannot view or reserve shifts. All shifts become private, and only administrators can assign people to them.
The pill on the row shows this as two icons, one for visibility and one for reserving, so you can read the applied state at a glance.
Lets approved users generate their own shifts on this opportunity instead of picking from shifts you created. A mentor logging planned meetings with their mentee is the typical case. The shift appears on the user's schedule and the coordinator is notified about it.
When Instant Book is on, an approved user who reserves a shift is confirmed immediately, with no administrative step. When it is off, reserving creates a shift request that you approve or decline.
This one setting changes how three other settings behave, so it is worth deciding early. See settings 8 and 9 below.
By default an approved user can reserve as many shifts as they like. You can cap that per user, counted either by shifts or by hours, and optionally only inside a date range.
No limit. The default.
Number of shifts. Set a minimum and/or a maximum number of shifts.
Number of hours. Set a minimum and/or a maximum number of hours.
A minimum is a commitment target rather than a block, so use the maximum when you need a hard ceiling. The date range decides which reservations count toward the limit, so leaving it blank counts all of them.
By default, users can cancel up until 12 hours before the shift start time. Your options:
Users can cancel up until 12 hours before the shift start time (the default).
Set a custom cancellation window, expressed in weeks, days and hours before the start time.
Users can cancel at any time prior to the shift start time.
Users cannot cancel once they are a reserved attendee. They have to contact you.
When someone cancels off a shift that was fully reserved, this sends the freed spot out to other approved users. A shift going from 5/5 to 4/5 is the case it covers.
Eligible means the user is not excluded by any of the following:
the reservation limit in setting 4,
the overlapping reservations rule,
already being a reserved attendee on that same shift.

Decides whether a user is asked why they are cancelling.
Not prompted. Do not ask for a reason.
Optional. Ask, but allow a blank answer.
Required. A reason must be provided before the cancellation goes through.
There is a dedicated article on this one, covering where the reasons show up afterwards: Using the Cancellation Reason Prompt on Vome.
Once you decline someone's shift request, can they ask again for the same shift?
Cannot request again.
Can request again.
Can request again if spots remain.
This applies only when Instant Book is off. With Instant Book on, users are booked right away, so there is no request to decline.
Two decisions about pending requests, read together on one row.
Show outstanding requests to users. Shows users how many spots on a shift have already been requested and are waiting for your approval. A pending request does not hold a spot, so without this a heavily requested shift still looks wide open.
Maximum outstanding requests. Stops a shift accepting new requests once enough are waiting, so a backlog cannot build up on one shift. Choose No limit, a fixed number of requests per shift, or a number of requests per spot on the shift. Approving or declining a request frees the space again.
Like setting 8, this only matters when Instant Book is off.
These settings govern what users can do on their own. Administrators are never restricted by them.
Setting 1 controls visibility and self-reserving separately, so you can show shifts without opening them up.
Instant Book decides whether a reservation is instant or becomes a request you approve.
Settings 8 and 9 are only active when Instant Book is off.
The spot-available notification needs cancellations to be allowed, or there is nothing to notify about.
Reservation limits can be counted in shifts or in hours, and can be scoped to a date range.