BitTorrent flags.txt

?: your client unchoked the peer but the peer is not interested
D: currently downloading from the peer (interested and not choked)
d: your client wants to download, but peer doesn't want to send (interested and choked)
E: peer is using Protocol Encryption (all traffic)
e: peer is using Protocol Encryption (handshake)
F: peer was involved in a hashfailed piece (not necessarily a bad peer, just involved)
H: peer was obtained through DHT
h: peer connection established via UDP hole-punching
I: peer established an incoming connection
K: peer unchoked your client, but your client is not interested
L: peer has been or discovered via Local Peer Discovery
O: optimistic unchoke
P: peer is communicating and transporting data over uTP
S: peer is snubbed
U: currently uploading to the peer (interested and not choked)
u: the peer wants your client to upload, but your client doesn't want to (interested and choked)
X: peer was included in peer lists obtained through Peer Exchange (PEX)