From:
http://www.alembic.com/info/wood_premium.html 3A Quilted Maple pictured on a Signature Standard bass. Quilted Maple gives a lovely brightness, with a distinctive 'shimmer' to the decaying sound. The figuring covers at least 75% of the face.
5A Quilted Maple pictured on a custom Series II 7-string bass. It has the same tonal characteristics as 3A Quilt, but the figuring is much more intense and covers at least 95% of the face. Each quilt is deeply incised and looks 3-dimensional, with sizes ranging from small coins to large capsules that span the entire width of the body.
These ratings for figure are not official in any way except by the person/body issuing them, so you can't always compare ratings between guitar manufacturers.
The National Hardwood Lumber Association has grading that is standardized, but not for figured woods. There's a little bit about their grading system
here