In my experience, you need to calibrate each guidebook or website to your own experience.
As a case study, I can speak from experience with two guidebooks describing hikes and treks in the same region:
- På Fjälltur: Abisko Kebnekaise, describes lots of trails in the area around Nikkaluokta and Abisko. They grade trails at a scale of 1–10. I've found routes graded as "7/10 difficult" to be quite easy, routes graded as "4/10 normal" to be very easy, and no route is graded below 3/10. Trails described as steep, I found no problem at all.
- På tur i Narvik og omegn, parts I and II, describe many trails and hike in the area around Narvik, just across the Swedish-Norwegian border from Abisko. Hikes are graded on a scale from 1 to 4. I've seen hikes described as "child-friendly" that I considered quite scary myself, with lots of boulders, steep slopes where losing balance is fatal, and wild rivers. I've made the mistake of trying a descent described as "steep", it was the most vertiginous descent I've ever done and would not recommend it to anyone loving their life.
I have some experience with these two guidebooks, so I know what I can do. There's no trail mentioned in the Swedish guidebook (På Fjälltur) that I would avoid on a backpacking trip, but for many of the trails from the Norwegian book, I would. Subjectively, on a 1-to-1 mapping, I would map any trails from the Swedish book rated 1–7 to be rated 1 in the Norwegian book, Swedish trails 8–10 rated 2 or 3 in the Norwegian book, with the Swedish book not containing anything rated 4 in the Norwegian one. Some routes are even described in both books.
This is just an example, but for me, it clearly illustrates that there is no universal set of guidelines. It depends very much on the standard and who one keeps in mind. It seems Norwegian kids are born on a a slope and they go mountain hiking in kindergarten on trails where 20-year old men from other countries would be scared. A Dutch hiking book may consider a route "difficult" because it's 20 km (even if all is paved and of course flat). So, no. There is no official grading or agreed upon set of attributes.