Final answer:
A yellow triage tag signifies a serious but non-life-threatening condition, prompting urgent medical care. In critical emergencies where the patient's blood type is unknown, Type O negative blood, the universal donor type, is transfused to prevent incompatible reactions.
Step-by-step explanation:
When a patient is tagged with a yellow triage tag, it indicates that the patient has serious injuries that require urgent medical attention but the situation is not life-threatening. In comparison, red tags are used for patients who must be treated immediately to survive, while green tags are used for the 'walking wounded' who have minor injuries. Black tags are used for patients who are deceased or whose injuries are so severe that they will not be able to survive given the limitations of the current situation.