First digit (HD5xx) is the range, higher number means higher end:
2xx, 3xx and 4xx are fairly cheap, closed-back and all-purpose. I believe the "Pro" variants have a fairly flat frequency response curve and can be used as a low-cost reference model.
5xx: Their mid-range, 'sweet spot' segment. I suppose one could call these the entry-level in the high-end headphone market. Very open and fun-soudning with excellent soundstage, quite an airy sound. Easy to drive at 50 Ohm.
6xx: A step over the 5xx line, warmer in the sound signature and supposedly more detailed. Also much harder to drive at 300 Ohms.
7xx: Somewhere between the 6xx and 8xx series. The only model currently is the HD700 which is sort of like a budget HD800 although the sound signature is said to be a little different (and actually better according to certain peoples' tastes) than their bigger brother. Half the price and half the impedance (150 Ohm) compare to the HD800.
8xx: Their flagship line. Need I say more?
Second digit (HDx5x) indicates position in the line, there isn't any definite naming scheme but it varies from range to range. Higher number means a higher model.
There's also some differences between different models in terms of sound signature. The HD558 is said to have more bass compared to the HD598 which has a more neutral bass response.
Third digit (HDxx8) isn't really relevant above the 5xx range as everything ends with a 0 and their older models where they use both the last numbers (HDx80) to indicate range. In their current line however it seems to indicate revision. For example they replaced the HD555 with the HD558, the number bump indicating it's a newer version of the HD55x.
Once again, this is just my interpretation and it's indeed bit fuzzy as they seem to have changed the way their naming scheme works a few years ago. I think the HD580 used to be the top of the HD5xx range (with the HD280 and HD380 being on top of the HD2xx and HD3xx lines respectively) before they replaced it with the HD590 and then the HD595. I might be entirely wrong on this though, so forgive me if some facts are incorrect.
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