This part of ISO/IEC 13066 defines the responsibilities of different information technology (IT) and assistive technology (AT) functional units in supporting interoperability. It recognizes that AT can be provided both as functional units that are installed or otherwise connected to a system or can be utilized by being provided as a service which is accessed via communications connections. It bases these responsibilities on fundamental IT definitions of major types of functional units. It focuses on the utilization of standard, public interfaces for functional units and on the provision of accessible documentation of their capabilities. This part of ISO/IEC13066 recognizes that IT is implemented both in conventional computer systems and as a major component of other systems within the wider scope of information and communications technology (ICT). This part of ISO/IEC 13066 recognizes the fundamental role of operating systems and application programming interfaces (APIs), in managing interoperability, and in providing guidance to developers of other functional units. It also recognizes that different operating systems will have their own standardized methods of supporting interoperability. This part of ISO/IEC 13066 does not define or require specific technology, commands, APIs, or hardware interfaces. It defers to other existing standards and supports the development of new standards in these areas. It identifies a variety of common accessibility APIs that are further described in other parts of ISO/IEC 13066.