A snapshot is a visual representation of the state of a PDF document at the time it was saved after an editing session. You save a snapshot to revert to a previous version of your PDF document. However, you do not necessarily have to save this snapshot at the end of each session. You can simply select any session from a list and save its snapshot at any stage in your workflow.
Consider the following example. You have created a Certified PDF document in which you use only black text and one spot color. The document has been designed to be printed on an offset press. At some point in the workflow, however, you also want to print this PDF document on a digital four-color press and therefore, you change the spot color into its CMYK counterpart throughout your entire PDF document. You may use Enfocus PitStop Pro, for example, to do this. This means that the latest version of your PDF document contains only CMYK colors. But, you also need to reprint this PDF document on an offset press using the spot color. You can then simply select a version which still has the spot colors, save its snapshot as a separate PDF document and send this PDF file to your offset printer.