The PTB Relation addon links 2 of your post types, created with the Post Type Builder, allowing you to query post type (A) while showcasing post type (B). It's perfect when you want to showcase a post, but also have some related posts that you think would interest your users as well. For example, when featuring a celebrity post type (A) you'll most likely want to showcase the movies that particular celebrity has starred in. With the PTB Relation addon, you can create a separate post type called movies and query these movies automatically while users are looking through your celebrity post type.