You can choose the Label plot's variable using the Variable menu under the Plot list the same way as you would with any other type of plot. One special property that distinguishes the Label plot from some of VisIt's other plots is that it can plot multiple types of variables. The Label plot can display information for meshes, scalars, vectors, tensors, array variables, subsets, and materials so you will typically find more variables available for the Label plot than you would for other plots. When you choose a mesh variable for the Label plot, you can display both the mesh node numbers and cell numbers otherwise you are limited to displaying only the variable being plotted.