Final answer:
Shares is the user-defined resource setting that is used to prioritize resource allocation during contention.
Step-by-step explanation:
The user-defined resource setting that only comes into play when there is contention for the resource is Shares. A reservation guarantees a minimum amount of a resource, a limit caps the maximum amount a resource can use, but shares are used to prioritize resource distribution during contention. When multiple processes or virtual machines are competing for resources, the one with a higher number of shares is given priority in resource allocation. Shares represent a relative weight or importance, and they only come into play if there is not enough resources for all concurrent requests.