Final answer:
A business case is the document that provides justification for investing in a project by outlining its value proposition and potential ROI, whereas a stakeholder register identifies and classifies the project's stakeholders for management purposes.
Step-by-step explanation:
The document that provides justification for investing in a project is often referred to as a business case or project proposal, not a stakeholder register.
A business case outlines the benefits, costs, risks, and rewards associated with a project and serves as a key document to convince senior management or investors to fund the project. It assesses the financial and strategic alignment of the project with organizational goals, which is critical for gaining support. It includes an analysis that justifies the project's value proposition and return on investment (ROI), as well as details such as projected revenues, cost savings, and market potential. In contrast, a stakeholder register is a project management document that identifies and classifies the project's stakeholders. This includes their interests, involvement, and impact on the project. It is essential for stakeholder management but does not directly justify the financial investment in the project itself.