Fields Required Validator
The Fields Required (Admin Essentials) validator blocks a workflow transition when one or more work item 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.
Workflow Expression Condition
The JQL / Expression Condition (Admin Essentials) hides a workflow transition button when the work item does not meet a Jira expression defined by the administrator. If the work item does meet the condition, the button appears normally and the transition can be executed.
Attachment Count Validator
The Attachment Count (Admin Essentials) validator blocks a workflow transition when an Attachment Field has fewer or more files than configured. Available from version 1.9.0 and exclusive to Jira Cloud.