A rough scale might be:

Mild ADHD: You're more distractable and forgetful than most. You have techniques to get around this such as reminders, planners, assistants etc.

Moderate ADHD: You struggle with important task completion all the time. You've experienced consequences from it such as job loss or dropping out of college You may have depression or anxiety related to your ADHD

Severe ADHD: You're in prison because of a violent outburst caused by your emotional disregulation. You're homeless because you can't hold down a job or a relationship. You're dead because you crashed your car while distracted.

It seems like a contradiction, but is not.

Many things are measured on a scale that goes from mild, to moderate, to moderately severe, to severe.

ADHD is not a true or false thing, which is what leads to the "everyone is a bit ADHD" myth.

Some people struggle in some situations, but with behavioural management can control it. Some people will find any excuse to not do what they're supposed to do, and have to fight the urge all day every day.

I personally had a bit of the latter, in that I often found myself just unexpectedly walking off mid work and finding myself across the building for no reason. It was just a physical urge to get up and do anything apart from work