Assigning single/multiple Posts
As the nature of WordPress Posts are flat (non-hierarchical), the assignment is straight-forward to the post itself and doesn’t include any other posts or categories that the post is related to.
The Post tab in the Assignment Panel provides you with the entire list of all existing posts in a flat, non-hierarchical form.
Assigning to All Posts
In order to assign a block to all posts, you should use the Auxiliary tab in the Assignment panel, and then check the ‘All Posts’ option. Checking this would apply the block to all posts – even those that would be added in the future. It’s like a live event that will always automatically assign new posts to the existing post set.