Difficulty Levels Explained

[theblob]

Where can one find an explanation of the difficulty levels listed on this site with sheet music?  Specifically, are the difficulty levels industry-standard in some way, or are they specific to the site?  Also, who assigns the difficulty levels?  It's not even immediately clear what the maximum difficulty level is, although I surmise that it's "8+".

I didn't find these things explained in the forums.

By the way, when a sheet music item is listed, it would be nice if one could, say,  mouse-over a difficulty level and have a question mark appear, such that when the question mark is clicked, one is taken to a page that explains these things.

[nilsjohan]

The levels are similar to the ABRSM grade system but the levels in our database are assigned by us (Piano Street). Everything above grade/level 8 is tagged 8+.

Help links in the form of question marks linked to appropriate sections of the FAQ page is a good idea.