The Associated Board of the Royal Schools of Music - (Associated Board or ABRSM for short) - is one of a number of British music examination boards which have defined a set of "Grades" for each musical instrument. Achieving a Grade means passing an exam.
A Grade represents what a typical pupil might learn in, let's say, 6 months to a year of one-to-one tuition.
The eight Grades, from Grade 1( lowest) to 8 (highest), define a musical progression (pardon the pun) that goes from novice to "equipped for Music School".
Associated Board is perhaps the UK's largest exam board in terms of setting up exams all round the world and is therefore an appropriate yardstick to measure the complexity of music - and in particular, since you're reading this - my ensemble music.
Remember that ensemble music is harder to play than solo music of the same complexity because of the extra amount of listening and adjusting of tempo and volume that has to be done on the fly. I've graded my pieces at the level that would suit someone with the equivalent ABRSM solo grades.
You can inspect the Associated Board Graded syllabus here
Note - ABRSM keep changing the lower-level addresses in their website - this takes you to the top level - search for Guitar when you get there
Associated Board also offer a set of Diplomas in Guitar. Each Diploma specialises in Teaching, Performing or Directing.
I teach private (one-to-one) lessons - if you want personal tuition & instruction on working towards a Graded Exam, please visit my Guitar Lessons website to find out more