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