PCB base copper weight vs plating thickness
Base copper weight or thickness is the amount of copper that is on the PCB before they do anything to it.

Plating is what they do to the board to finish it, which also fills in the vias and holes with a layer of (extra) copper. And thus the total thickness Saturn reports, as some simple maths would show you, is just the two thicknesses added together, that will be the final thickness of your outer copper.

If you have 1oz base copper and plate 1oz onto it, you get a total of 2oz on the outside. 

It is important to note that plating only affects the outer layers, you will also not be allowed in the Saturn Tool to add plating for embedded traces.

Usually when a fab specifies a thickness they only specify the finished thickness, and in case they specify 1oz/1.4mil/36&mu;m it is most usually 0.5oz base thickness and 0.5oz plating thickness. More specialised and/or professionally aimed fabs, or most fabs at special request, will tell you the exact numbers for both.

The tool uses these parameters to estimate the edge coupled capacitance of traces as well as skinning effects in some tabs. In most cases it's more important to get the end total as close as possible, but there may be a few tabs that use some side-effects of plating, though I have not noticed any sections accounting for those. That can of course also change in future releases, they do refine the maths on some things once in a while.

So in your specific case the most likely/best option is to set base to 0.5oz and plating to 0.5oz.