SeM Rush - keyword difficulty numbers

Just signed up to SEMRUSH through a group buy. In terms of the keyword difficulty what is regarded as easy, medium, difficult etc. Can someone give me ranges please. e.g.

Is it 0 - 40, 50, 60 is easy ??

Also, assuming that if a keyword is showing as zero difficulty this is simply a mistake or there aren't enough searches?

is there a balance between searches and difficulty which makes it worth going after? For example k/w difficulty divided by searches should be above/below a certain number?

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Really thought someone would have had an answer to this. Perhaps I should phrase it differently. Everyone on here talks about low, medium, difficult etc keywords. How do you establish what is in each category?

I belive semrush displays the competition in terms of %, but I really think that you are forgetting one crucial thing in your question, the level of kw competition also depends of the skill set of the seo, what some seo's would consider low competition others might regard as medium

Well, I guess try to judge for yourself. Scale is 0-100 (%),so if you have access to another keyword tool you are familiar with cross check a few and see what qualifies as low, med and high comp.