The label chip next to the Document ID is showing the current status of the invoice. If the status is Assembled, you can change the status to:
Back to Pending (dissolve the invoice), Ready for Posting or Posted. When the status is Ready for Posting or Posted you can not change the status. If the status is set to Pending the invoice will be dissolved, and the corresponding document lines can be found in the corresponding Pending list.
If you click on a document line, you will se more information for the specific document line.