Number of bytes pending to be sent. This is data that you have recently requested to be sent but has not yet actually been put on the wire. The reliable number ALSO includes data that was previously placed on the wire, but has now been scheduled for re-transmission. Thus, it's possible to observe m_cbPendingReliable increasing between two checks, even if no calls were made to send reliable data between the checks. Data that is awaiting the Nagle delay will appear in these numbers.
Number of bytes of reliable data that has been placed the wire, but for which we have not yet received an acknowledgment, and thus we may have to re-transmit.
High level state of the connection
Connection quality measured locally, 0...1. (Percentage of packets delivered end-to-end in order).
Packet delivery success rate as observed from remote host
Current data rates from recent history.
Current ping (ms)
Estimate rate that we believe that we can send data to our peer. Note that this could be significantly higher than m_flOutBytesPerSec, meaning the capacity of the channel is higher than you are sending data. (That's OK!)
If you asked us to send a message right now, how long would that message sit in the queue before we actually started putting packets on the wire? (And assuming Nagle does not cause any packets to be delayed.)
Internal stuff, room to change API easily
Quick connection state, pared down to something you could call more frequently without it being too big of a perf hit.