According to the ServiceNow documentation1, a user's availability is calculated as the ratio of allocated hours to total capacity, expressed as a percentage. Allocated hours are the hours that a user is assigned to work on a project or demand. Total capacity is the maximum number of hours that a user can work in a given period, based on their schedule and availability profile. A user's availability indicates how much of their time is already allocated and how much is still available for new assignments.
1:
https://docs.servicenow.com/bundle/vancouver-it-business-management/page/product/resource-management/con