Currently there are two resolution mechanisms for extra shift requests.
1. First to Respond
This means that after you send the request to 1 or more people, who ever click on the 'Accept' button first will be the one receiving this shift.
After you send the request to 2 or more candidates, they are allowed to "express their interest" in getting the shift by accepting the request, which will remain Open until same manager who created the request or someone with sufficient rights, comes in and selects who will get the shift.
You can review contract statistics and balances before deciding and you can assign the shift to 1 or more people that have accepted.