What technique is used by a business analyst to document user interaction with a proposed system involving primary, alternate, and exception flows?

The technique used to document user interaction with a proposed system, including primary, alternate, and exception flows, is scenarios and use cases. This approach provides a structured method to describe how users will interact with the system in various situations, outlining not only the main path that a user might follow while using the system but also alternative paths and exceptions to those main paths.

Use cases are particularly valuable in capturing the functional requirements of a system from the user's perspective. They detail the interactions between users (actors) and the system, making it clear what the system should do in response to specific inputs or events. By outlining primary flows, which represent the most common path a user would take, along with alternate flows that cover variations and exception flows that account for error conditions or unusual but possible events, business analysts can ensure comprehensive documentation of user requirements.

In contrast to the other techniques mentioned, user system interface focuses on UI design elements, user stories are typically high-level descriptions used in Agile methodologies that may not cover all specific scenarios in detailed format, and goals and accomplishments generally focus on overall outcomes and success metrics rather than the detailed interaction sequences that use cases provide. Thus, scenarios and use cases are the most appropriate technique for detailing user interactions in a systematic manner.