strange yellow square

Yesterday I was taking a look at my missy elliott CDs when I noticed a strange looking yellow square on the top of the spine on three of the CDs. Those three CDs where ''Under Construction'', ''This Is Not A Test!'' and ''The Cookbook''. It's funny because those where all released in that order. Those squares are not on ''Da Real World'' or ''Supa Dupa Fly''. As for ''Miss.E...so addictive'', I wouldn't know. ''Miss.E...so addictive'' is the only missy CD that I don't own. What do these yellow squares mean? Why are they not on her first three but on her three others?