Esvelt, K. M. et al. Orthogonal Cas9 proteins for RNA-guided gene regulation and editing. Nat. Methods 10, 1116–1123 (2013).

In this publication, Esvelt and colleagues described a set of Cas9 orthologs that are orthogonal to the widely used Strptococcus pyrogenes variant. These could be used to regulate sets of genes independently and simultaneously.
'Low' and 'High' here represent the gene expression levels with either mismatched or matching spacer sgRNAs being expressed.