I couldn't get tooltips to show. But here's what they seem to be:
A - click to see Address information (all kinds of contact info in there)
N - indicates it's a free 'Net' TV channel (streamed for free on the Internet)
U - click to see info on that transponder's Uplink station
F - indicates it's a Free satellite channel
P - click to see info on the Package of which that channel is a part, on that satellite