Final answer:
The requirement for a porter to wear a uniform depends on the policies of the establishment they work for, and exceptions may apply under certain conditions such as being off-duty or during emergencies.
Step-by-step explanation:
The question asks about the circumstances under which a porter need NOT be in uniform. Without additional context provided by the question (such as a specific rule, policy or law that mandates a porter to wear a uniform), it's difficult to provide a definitive answer. However, generally speaking, the requirement for a porter or any employee to wear a uniform would depend on the policies of the establishment where the porter works. If the rules specify that uniforms must be worn during working hours or when performing certain duties, then the porter would need to comply with this policy. On the other hand, there may be exceptions such as when the porter is off-duty, when they are traveling to and from work, or if there is an emergency situation where donning a uniform isn't practical or required.