The use of a trigger condition is optional. If no test conditions are specified, the trigger will be unconditionally invoked. No conditions are allowed if "Open" or "Close" Trigger Points are used.
If the trigger is defined in the field, the conditions can only involve the field to which the trigger is being linked. If the trigger is defined at the table level, the conditions can involve any other real or virtual columns in the table to which the trigger is being linked.
Trigger Concepts in the Developer Guide.