The internal interfaces of a system or subsystem are documented in their respective interface specifications, while human-machine interfaces can be included in a system design document (for example. B a software design document). It can usually contain (or refer to) battery boundary drawings, an interface matrix, or an interface register. Critics of requirement documentation and system engineering in general often complain about the extra cost of documentation.   CIIs are often present in document-controlled projects, but can also be useful in agile projects (although they are not explicitly designated as such).   A DCI is not necessarily a text document. It can be a (evolving) table of Goes-Intos and Comes-Outs, a dynamic database that presents each subsystem as a DB view, a series of interaction diagrams, etc.