Welcome to FS, bare with my long reply, but I don't think this is a question with a single answer.
I have historically really struggled with restricted HR training. I have only been moderately fit for about 1/4 of my life and only after 17 (hence my modest PBs despite serious training in my early 40's). Recently I have persevered on trying to find a pace I can maintain for long (60-105 min) without breaching 75% HRmax. This has been painfully slow, but over the last 4 months I have seen my HR drift slow, so while I needed an average HR of about 68% in August to keep below 75%, I can now manage an average of 70%. I hope that this isn't largely due to cooler conditions! Also this is at 2k pace +30S now (dropped about 2S since May in addition to 2S knocked off 2k pace). Initially I struggled to row slow enough without allowing my stroke to become sloppy (this is now 17SPM, was less than my initial 16SPM pace!). As a result, I am a long way from being able to follow Jon's suggested stroke rate. Some plans suggest ignoring HR and just go by rating. I used to do my "slow" sessions at 10k pace + 10S, but always had relatively weak threshold rows (5 & 6k), so I think that the HR directed rows are useful. In addition, as we get older, we either need more days of rest or to make the easier rows easier, UT1 rows no longer suffice for me.
Sorry for the long pre-amble. I have long sort a definition of Aerobic Threshold. Cyclists have defined it for fit cyclists as the pace you can maintain for about 8 hours! This does not convert to rowing. Anyone who has attempted a 100km row knows that this is not limited by theoretic physical fitness but by mental endurance and the limiting factor is enduring the pain in your glutes, hip flexors and quads. I have seen people advertising to measure aerobic threshold in a lab, I would really like to know what they measure and whether this is universally accepted. Joe Friel (author of the well respected "Fast After 50" as well as leading 70+ cyclist, coach and triathlete) defines it as 60% of the Watts of your anaerobic threshold (broadly 2k pace), but he goes on to use 30 BPM less than anaerobic threshold. Given the wide range of HRmax, this would mean it is proportionally much lower for those with a lower HRmax that is unlikely. Still others use the proportion of energy from fat rather than carbs (couldn't find a definitive definition, but probably where fat >50%). So I have assumed that this is a relatively vague divide. Personally I think the importance of the HRcap used is that it allows you to recover sufficiently from hard sessions to do the next hard session. Clearly the hardest sessions take full rest days to recover from, but regular sessions for those doing >4 sessions a week need to be possible after a day or 2 of slower rowing. As a result, I would monitor how well you feel you have recovered. I think this depends upon how often you wish to row and how long you are happy to leave between hard sessions. Call this UT2 if you wish, but it is only a label. The important point is that it is right for you.
Going back to the question, HR capacity is affected by BPM and stroke volume. Both increase as we increase exercise intensity and the way they do varies between individuals. As such "%HR zones" are only a guide. For example, Anaerobic Threshold is where the body starts to struggle to avoid acidosis and accepts a decrease in blood pH before starting to increase the rate of blood circulation. For most it is also where a larger increase is from stroke volume than HR so there is an inflection in the curve measuring how HR increases with workout intensity. I have not come across a clear physiological marker for the Aerobic threshold. Many fit endurance athletes can have an anaerobic threshold at >90% of HRmax, so the assumption of 80% often used is not universally applicable. I suspect the same would be true of any of the aerobic threshold limits. I believe the top of UT2 is the aerobic threshold as the top of At is the anaerobic threshold, so it is not worth getting worked up over the precise numbers of the zones. More important is that HR varies with stress, illness and tiredness so an HR limit is a better way to limit training than a particular pace.
Hope that helps. For any of the more knowledgeable, please let me know if I have misunderstood anything or they can help reduce my confusion.
As for how to use the zones, Joe Friel recommends training at a constant speed on slower sessions (or possibly speeding up), so you should aim to row at a pace at which you can row for the required duration to finish approaching but not exceeding the limit. I find my HR increases markedly at a point where I start to sweat freely (7-15 min in depend on temperature and intensity for slower sessions). I would think this should be within the band (ie above the minimum). I allow 2SPM for a few beats as this is transitory and allow a bit of leeway if a non-exercise factor causes it (eg being disturbed by something going on, sneezing, needing to go to the loo etc.) No idea whether this is the norm.
Let us know how you get on.
52 year old Lwt (in ability and weight) trying to develop a technique that doesn't cause hysterics and start rowing regularly now I am working again.