Your site has a defined seniority order in place that gets used when multiple people request the same Extra Shift.
For example, if you have one Extra Shift available and 3 people all said they could work it by submitting a claim request, then those 3 requests will appear together in the Pending Requests tab, ordered by seniority, starting with the most senior person at the top.
To start, go here (Admin Tab -> Employees -> Seniority Order)
The screen you arrive at will list all of the users on your site, in seniority order, starting with the most senior at the top.
To change this order, simply click & drag users into their new positions. This ordering saves on-the-fly, so once you have the order you want, you can just leave the page.