Quickplay Letter Code

[MusicGamer]

Hello everyone,
I have a little question about the "quickplay" menu.
For each song, we can see in order:

A logo showing if the song is downloaded or not
The name of the song
The best star rating
A little letter
The high score

Does somebody know what does the little letter mean?

Sincerly,

[SirDavidTLynch]

It indicates the difficulty played.

[MusicGamer]

Thank you!