Saving and exporting are different. When you Save your Prism file you save it in Prism's file format. This type of file can only be opened with Prism and it contains data, analyses and graphs.

Use Export to save a graph or layout in an image format that can be imported to another application. If you export a graph or layout, you are simply creating a picture of it. You won't export the original data or any analysis results. You won't be able to open the exported file or edit it, except as a picture. If you export a data table as a text file, you are just exporting the values.