Target utilization is the percentage of hours that a user is expected to work on billable tasks/projects compared to their available hours (capacity). A resource manager specifies target utilization for individual users in user’s profile.
Billable utilization (against capacity) is the percentage of billable hours reported by a user for assigned work divided by user’s capacity. User’s capacity is based on user’s calendar availability.
Billable utilization (against business hours): is the percentage of billable hours reported by a user for assigned work divided by working hours defined in the organizational calendar.
Resource utilization category is a custom field (picklist) for work items used for categorizing different types of work. Each defined category appears as a separate column in the Resource Utilization report.