Business Context
Identify all neighbouring systems and specify all logical/business data that is exchanged with the system under development. Add data formats and communication protocols with neighbouring systems and the general environment if these are not specified in detail with the relevant components.
More help to document quality goals in this overview is found on docs.arc42.org.
Using projectdoc
A context is typically documented with a View document. You may transclude from this document any sections that suites the information needs of your stakeholders.