The concept of function is concerned with how the stakeholder groups use the solution from input, process, and output perspective. The business analyst will consider the inputs that the stakeholders provide to the solution's processes, and how they then use the solutions processes to generate specific outputs.

Let's consider the following real-world example: 

A stakeholder group might have a primary function of analyzing daily sales for a company. They use a daily sales report to execute their tasks. If a new solution provides a new report format as part of a change and this doesn't include the team's information requirements to perform their daily sales analysis, then this will impact their group's function directly and should be included as part of a stakeholder impact analysis.