Instead of using the historic word “commonplacing”, Eminem uses the fantastic euphemism “stacking ammo”. Given his use of his words and lyrics collection in battle rap, this seems very apropos.
Cooper analogizes the collection as the scrawlings of a crazy person. In some sense, this may be because there is no traditional order, head words, or indexing system with what otherwise looks like a box of random pages and ideas. One might argue that the multitude of notebooks, papers, colors, sizes, etc. provides a sort of context which Eminem could use as a method of loci for remembering where to find particular ideas, thus making the need for an indexing system feel superfluous to him. This is even more likely if he’s regularly using, maintaining, and mining his material for daily work.