transmission/docs/Peer-Status-Text.md

The cryptic 'peer status' is shown in transmission-remote's --peers argument and in the Peers tab of the GTK+ Client's Details dialog. It's explained in a tooltip if you have GTK+ 2.12 or higher, and here if you don't:

O	Optimistic unchoke
D	Downloading from this peer
d	We would download from this peer if they'd let us
U	Uploading to peer
u	We would upload to this peer if they'd ask
K	Peer has unchoked us, but we're not interested
?	We unchoked this peer, but they're not interested
E	Encrypted Connection
H	Peer was discovered through Distributed Hash Table (DHT)
X	Peer was discovered through Peer Exchange (PEX)
I	Peer is an incoming connection
T	Peer is connected via uTP