Hi,
The picture did not work but I If you mean what I think you mean then no, S does not refer to the slice number.
RAS which means (Right Anterior Superior) is the coordinate system in 3d slicer it is the same as the XYZ coordinate system of the CT scanner. See more about coordinate systems here
The origin of the scan 3D coordinate system is set up by the CT operator who will zero the machine in a certain position. If the S says 155 then this means the slice you are looking at is 155mm above the position where the operator zeroed the CT machine.
If you want to see which slice number you are at then you need to look at the image coordinate system or IJK coordinate system the K coordinate should tell you which slice number you are at. Assuming your scan is axial then hover your mouse over the red slice and look at the number shown in the following picture.
But remember that slicer uses zero indexing for the K coordinate so the first ‘slice’ number is 0 so if you are looking for slice 21 then you may need to go to slice number 22 in slicer.