ContentPlaceHolder in master page is reference in Content control in content page
Content control's Id can be empty, but the ContentPlaceHolderID need to be filled. The position of Content control in content page does not matter.
In ContentPlaceHolder, you can fill in some default content. But they will be override once content page reference it.
If you use nested ContentPlaceHolder and you want to fill the content, its parent ContentPlaceHolder can not be reference, otherwise, the content in nested ContentPlaceHolder will not be used.