What does the P stand for on the unilden bc144xl bearcat scanner?

It probably stands for priority channel. Most scanners have a priority function. I have a Yupiteru MVT-7100 scanner, and can set channel 1000 as priority. when I enable the function - it puts a P in the display. I'm guessing it's a similar for your model. Basically the priority function will check the programmed frequency every couple of seconds to see if there's any activity. If there is, it stays on the frequency - if not, it continues scanning.