Fields Required Validator
The Fields Required (Admin Essentials) validator blocks a workflow transition when one or more issue fields are empty at the time of execution. It's useful when a workflow state requires minimum information to move forward (for example, "Resolved" without a root cause, or "In Review" without an assignee).
Field Dependency Validator
The Field Dependency (Admin Essentials) validator blocks a workflow transition when a target field is empty while another field (the trigger) holds a specific value. It is the natural way to model "if A happens, then B is required" relationships without making the field mandatory in every situation.