What is a CAG::ChR2 rat?
CAG is a synthetic, ubiquitous promoter which combines elements of the CMV early enhancer, the chicken &beta;-actin gene, and the splice acceptor of rabbit &beta;-globin. According to the researchers who created it:


  The elements which constitute the pABWN vector, the CAG promoter, the weak neo unit, and the BPV fragment have only low specificity to cell types or species. Therefore, this system can be applied not only to L cells or CHO cells, but to a wide variety of cell lines. The pABWN vector should be very useful for producing large amounts of valuable proteins in mammalian cells, for changing the phenotype of host cells by overexpression of some normal or mutated genes, and for disrupting the function of targeted genes by overproduction of anti-sense RNAs.


In the paper you cite, CAG-ChR2 is used to activate all neurons in the vicinity of the light stimulation, for example in Figure 2Ab, where the panel is labeled "PYR+INT activation."