The dimension (or index) of the data hyper-cube which is not to be used as loop-index when plotting all the selected data. Usually corresponds to X-axis selection, i.e. it is "pt" for "Time", or "ch" for channel/wavelength, etc. If plotting visibilities vs channel index, for example, selecting "ch" as slice would produce one plot per scan, unless the "All in 1" option is selected.