ISO 18441:2016 defines the Application Program Interface in terms of:
a) the components that provide the services of the API;
b) the functionality provided by each of the components;
c) the interfaces provided by each of the components; and
d) the externally visible behavior associated with the interfaces exported by the components.
It does not specify:
ISO 18441:2016 defines those aspects of the Application Program Interface, which are common for all SLE service types or for a subset of the SLE service types, e.g., all return link services or all forward link services. It also defines a framework for specification of service type-specific elements of the API. Service-specific aspects of the API are defined by supplemental Recommended Practice documents for SLE return link services (references , , and ) and SLE forward link services (references  and ).
ISO 18441:2016 for the Application Program Interface responds to the requirements imposed on such an API by the CCSDS SLE transfer service Recommended Standards that were available when this Recommended Practice was released.