A project schedule lists a resource as 50% because it likely:

In project scheduling, resource allocation percentages reflect how much of a resource's total availability is assigned to a given project or task. A 50% allocation means the resource is splitting their time between this project and at least one other project or responsibility. This is a standard way project management tools like MS Project represent shared resources across multiple work efforts.

A resource shortage would be indicated by unassigned tasks or missing resources on the schedule, not a specific allocation percentage.

Overallocation occurs when a resource is assigned more than 100% of their available capacity, typically flagged as a scheduling conflict.

Underallocation describes a resource being assigned less work than their available capacity, but 50% specifically communicates intentional sharing, not a gap in utilization.