The project implements Measures 1, 4 and 8 provided for in the Updated Roadmap for the Implementation of the Updated Strategy for the Development of eGovernment in the Republic of Bulgaria 2019-2023. (ASREU), which complement each other. A Reference Architecture for Interoperability (RAA) will be developed, which will be part of the updated eGovernment Architecture (EGovernment Architecture). A plan for implementation of the PAH will be developed and approved, including the preparation of proposals for regulatory changes related to its implementation. An Information System for centralised construction and maintenance of registers, meeting the requirements of the EA and corresponding to the philosophy of the ACER, will be established and implemented. The IS will provide the possibility of integration with the layer for the exchange of credentials and reference information and will comply with the EA, so that the information in the registers is accessible and up-to-date, presented in formats that can be machine-processed without the need for prior transformation. Through the IP, it will be possible for each agency to automatically set up a data structure and execute work processes related to the services provided through a register, as well as graphical user interfaces to access the relevant services. It will be possible for an administration to keep the data in multiple registers for which it is a primary data controller (PAD), in isolated spaces (tenants), as well as multiple administrations to keep 1 or more registers in an isolated space provided by the IS. The IS will automatically generate web services necessary for interaction with external systems, including through the integration bus for the exchange of credentials and reference information (RegiX). The interoperability registers set out in the EAEC will be established through ICPR and the applicable data models will be recorded/migrated. The project will build a minimum of 4 MSD registers and a single Employment Register for which the PAD is the IA.