Documentation:Embed

Embed (e) at an association end is best explained as "in which table is the foreign key placed" This is often self-evident:

One-to-Many - foreign key will go in many end. Many-to-Many - foreign key will go into link-class(possibly implicit) One-to-One - This is where Embed-flag becomes important, because it is optional where to put the foreign key. If you do not know what end to pick - you can just pick any of them. But as you grow more experienced you might want to have the ability to control this.

If you set Embed=false on the many-end you prevent the framework from doing the reasonable thing - the framework will then assume you have a good reason and create an implicit association class where it can but the keys.