Key Docs are versions of a document that save annotations on the document, but those annotations are always editable. You can move or delete a callout, or clear the annotations (including redactions) on a Key Doc.
A Snapshot makes a flattened image of what is displayed, including any annotations or callouts, and this image cannot be altered. Snapshots are useful if you want to import a document with a callout into Word to include in a motion, or have a document with a callout in PowerPoint.