SAP S/4HANA 1511, on-premise edition 1511 is out now (see the announcement http://news.sap.com/sap-delivers-massive-wave-of-innovation-for-sap-s4hana/). It contains a first set of extensibility features for key users. In other words, key users can extend existing SAP Fiori apps within the SAP Fiori UI itself.
You can find a general overview on the extensibility features for key users in the following document The Key User Extensibility Tools of S/4 HANA, and a general introduction to S/4HANA extensibility in S/4HANA Extensibility – The new White Paper.
The first key user extensibility features that are available with SAP S/4HANA, on-premise edition 1511 are UI flexibility, custom fields and custom logic.In the following user assistance document you can find the feature description of the SAP Fiori application Custom Fields and Logic:
http://help.sap.com/s4hana_op_1511 -> section: SAP NetWeaver for SAP S/4HANA, on-premise edition -> General Functions for Key Users.
or follow this link.
As a first starting point, you might want to check out the short video that you can find in section Extensibility ( link ).
For SAP S/4HANA, on-premise edition 1511, the extensibility tools for key users have to be set up and connected to the adaptation transport organizer (ATO). The necessary steps are described in section Setting Up Adaptation Transport Organizer in chapter App-Specific Implementation Information under the heading Implementation Information ( link ).
Note: Please see also the following SAP Note that contains the corrected list of components that are required for using the key user apps, and a corrected list of ABAP statements that are available with ABAP for key users in SAP Note 2212658.
If you want to check out the feature description for S/4HANA 1511 cloud edition, follow this link: link