What did Kellogg mean by calling the sole survivor the backup in Fallout 4?

In Fallout 4, Kellogg refers to the Sole Survivor as "the backup" because they are not the original target of the kidnapping and assassination plot involving the game's primary antagonist. The term implies that the Sole Survivor was an unexpected participant in the events that unfolded, essentially serving as a secondary option or alternative to the initial target, which adds layers to the narrative about identity and purpose in the post-apocalyptic world. This characterization underscores the themes of fate and agency within the game's storyline.