Final answer:
The PSH flag in a TCP packet instructs immediate data processing rather than clearing the buffer, while the URG flag indicates urgent data that needs to be prioritized.
Step-by-step explanation:
A TCP packet using the PSH (Push) flag signals the receiver to push the data to the application as soon as possible, rather than buffering it. This does not clear the buffer but rather instructs the receiving end to process the data immediately. On the other hand, a TCP packet with the URG (Urgent) flag indicates that there is urgent data within the packet that should be processed immediately by the receiving application, preempting data that is not marked as urgent. The use of URG does not directly relate to buffer clearing but to the prioritization of the data contained in the packet.