In the July 2017 release you will see the invoice approvals workspace is available. This is a good addition to replace a function that was missing since the move from AX2012 to have some mobile function to approve an invoice while out of the office. While you could log in with a browser and approve this isn’t really convenient on a mobile device like a phone. The invoice approval workspace has some benefits seeing it can cache data and then sync back so as long as the data has been synced in the app you can review it off-line.
Here is a quick overview.
D365 July 2017 Update 8 (7.0.4565.16211)