When you choose "Normalize to > Total" on the Peak Area Replicate Comparison graph, each of the bars in the bar graph are stretched so that they have the same total height. This makes it easier to visually compare the relative heights of the segments of the bars to each other, to see whether the transition area ratios are consistent across all of the replicates.
When you choose "Normalize to > Total Ion Current", the values that would have been displayed on the graph are divided by Total Ion Current Area (TIC Area) value which was recorded when Skyline extracted chromatograms from the raw file. (The values are also multiplied by the median TIC Area across all the replicates so that the magnitude of the displayed values is similar to they were before normalization, but have also been scaled by the individual files TIC area).
The TIC Area is calculated by integrating the total signal in each of the MS1 spectra in the raw file across the entire gradient.
You can use the Document Grid to see what the Total Ion Current Area value is for each of the Result Files in the document.
You can learn about the Document Grid here:
https://skyline.ms/wiki/home/software/Skyline/page.view?name=tutorial_custom_reports
Here is a page that talks about normalization methods (although it does not mention "Total" because that's just a visual thing on the bar graph)
https://skyline.ms/wiki/home/software/Skyline/page.view?name=NormalizationMethods
-- Nick