This document defines the functional architecture of the Data Space for Digital Twin applications (DSDT) layer at the top of the IOWN technical infrastructure. DSDT encompasses functional blocks that enable the implementation of the digital twin applications allowing and easing the exploitation of the IOWN technical infrastructure. The functions span the data management and integration for the Digital Twin, the support for the data analytics to model the real asset behaviour in the digital world, and the interaction with user through manipulation of the Digital Twin.