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 is explained in a tooltip if you have GTK+ 2.12 or higher, and here if you do not:

`O` | Optimistic unchoke
`D` | Downloading from this peer
`d` | We would download from this peer if they would let us
`U` | Uploading to peer
`u` | We would upload to this peer if they would ask
`K` | Peer has unchoked us, but we are not interested
`?` | We unchoked this peer, but they are 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