When you load data into an app you analyze the data to get new insights. You can present your new insights in a new story or in a edited version of an existing story.
A snapshot's state and selections will not be updated at a data reload. It will always reflect the data that existed at the point in time the snapshot was taken. However, live data sheets are not static and will be affected by a data reload.
You can replace a snapshot of a visualization on a slide by going to its source, in the app. The visualization will now have updated data. You can take a new snapshot to use in your story.
When you take a snapshot you can make an annotation for your snapshot. The annotation helps you distinguish between the different snapshots in the snapshot library when you build your story. The annotation is not visible when you play the story.