Understanding who is filling the standard roles of clients, project manager, sponsor, and team members is generally enough for smaller projects. On small projects, there are not as many people involved and the work is usually not very complex. However, as your project gets larger, it's important to allocate people into more specialized roles. These specialized roles help to make sure that important aspects of the project success have more focus.
Please note that even on large projects, the following roles don't have to be full-time (although they could be if your project was large enough). For instance, it's possible that a person could have the role of a Business Analyst, but also spend 20% of their time in the role of Risk Manager.
Client Project Manager
If the project is large enough, the client may have a primary contact that is designated as a comparable project manager. As an example, if this were an IT project, the IT project manager would have overall responsibility for the IT solution. However, there may also be projects on the client side that are needed to support the initiative and the client project manager would be responsible for those. The IT project manager and the client project manager would be peers that work together to build and implement the complete solution.
Once you establish a document repository for a large project, you'll want someone to be responsible for managing it. The Librarian is responsible for establishing, maintaining, and enforcing document repository standards and monitoring them for conformance. They are responsible for all aspects of document management on the project.
On a large project, risk management may be a significant part of the project manager's work and it may be advantageous to appoint someone to coordinate the risk management process. The project manager retains overall responsibility for project risk but the risk manager would control the process of identifying, classifying and quantifying the risks and for monitoring the Risk Management Plan to ensure it is successful.
On a large project, quality management could take up a large amount of project management time. Under the guidance of the project manager, this person would write the Quality Plan, develop the quality control and quality assurance procedures, check that these procedures are being followed and provide advice and guidance to team members on quality related issues.
Suppliers and vendors usually refer to third-party companies or specific people who work for third parties. They may be subcontractors that are working under your direction or they may be supplying material, equipment, hardware, software or supplies to your project. Depending on their role, they may need to be identified on your organization chart.
A Steering Committee is a group of high-level stakeholders that are responsible for providing guidance on overall strategic direction. They don't take the place of a Sponsor, but help to spread the strategic input and buy-in to a larger portion of the organization. The Steering Committee is usually made up of organizational peers and consists of a combination of direct clients and indirect stakeholders.