"Edit in New Studio"
There are some features in the new Studio that are not fully compatible with Classic Studio. For this reason, if you create content in the new Studio, you will be able to see it in Classic Studio, but you must switch back to the new Studio to edit the content that was created in the new Studio.
Clicking on Edit in New Studio will take you to the Compose page for the campaign in the new Studio.
"Edit in Legacy Studio"
Editing content created in Classic Studio in the new Studio is currently not supported. If you try to edit content created in Classic Studio in the new Studio, you will be prompted to edit the content in Classic Studio.
Clicking on Edit in Legacy Studio will take you to the editing page for the post in Classic Studio.