You can filter Views Queries of hierarchical post types by Post Parent.
WordPress has an inbuilt post hierarchy concept which allows you to set individual posts as parents of other posts of the same type. The native pages post type is an example of this, and when you register new post type with Types you can specify if the post type is hierarchical or not.
The post_parent field is a standard WordPress field stored in the wp_posts table which contains the post id of the parent post (zero if there is no parent).
When you add a Parent Post query filter it is this field that is being compared and you will see the following options:
No parent (return top-level elements)
The post_parent field is zero, meaning the post has no parent.
Parent is the page where this View is shown
The post_parent field matches the id of the page where the View is inserted.
Parent is the current post in the loop
The post_parent field matches the id of the current post in the loop, used with nested Views when listing a series of posts to display content from their immediate descendants.
Parent is:
Used to list child posts of a specific post.
Post with ID set by the shortcode attribute
Used to list child posts of a specific post where the id of the parent post is set by a shortcode attribute so that the same View can be reused for different posts.
You can specify the attribute name, though Views will suggest wpvchildof, by default. The attribute name used in the filter needs to match the attribute added to the wpv-view shortcode where the View is inserted.
Post with ID set by the URL parameter
Used to list child posts of a specific post where the id of the parent post is set by an URL parameter so that the same View can be reused for different posts.
You can specify the URL parameter name, though Views will suggest wpv-child-of, by default. The parameter name used in the filter needs to match the parameter added to the URL for the page where the View is inserted.