Purposes are only a part of build management. The release and purpose combination for a project determines which process rule the project uses.
Each release contains a list of process rules that are valid for that release. Use process rules to control the process for a team for a particular release, and to let teams working on different releases use different processes.
A process rule specifies how a project is updated. To use a process rule, a project must have a release and purpose set. There is one active process rule for each purpose for a release.