Explanation

Box 1: The field is appended to the end of the existing section.
When you add new elements to a form that is to be merged, we recommend that you include your new elements within new container elements (tabs or sections). Additions to any container will be appended to the end of the container. For example, fields added to a section will be positioned at the end of the section.
Box 2: The field is added in a new section.
Box 3: The field is inserted in the middle of the existing section
Reference:
https://docs.microsoft.com/en-us/previous-versions/dynamicscrm-2016/developers-guide/gg309329(v=crm.8)