How do we define horse training “abuse”?

When do horse training methods cross the line from “accepted” to “abuse”? As with all ethical issues – your truth, my truth …whose truth?
Equitation science seeks objective ways to understand horse behaviour and learning. In this article experts weigh in. My summary, from this article:
1.How people define “abuse” depends on their culture, riding traditions, perception.
2.We can only draw the line when we understand objectively, the horse’s ability to cope physically and emotionally with the environment, our cues and the demands of training placed on them.
3.Something to consider… while competition rule books call out excessive spurring, jerking, tying and various training tack as abusive, “accidental abuse” may slip under the radar. Abuse by conflicting aids and confusion. I see it at many horse shows. These riders don’t mean to abuse their horses. They just don’t understand how horses think and learn. Some, “kill them with kindness”. They forget that horses are horses. Horse welfare depends on consistent boundaries. If not, they can become dangerous to handlers and the dread of equine vets!