Permission schemes define project-level permissions. Unlike global permissions, which can only be granted to groups, these can be granted to specific users, groups, project roles, and more. The option you choose here will depend on your use case. Generally speaking, groups will be the most straightforward option, as users often belong to one or more groups and you can easily model your permission requirements based on groups. Other options are useful when you have special or edge cases where you need to accommodate users that do not fit into your group setup. Once the permission scheme is configured, you can apply the scheme to individual projects. In this way, different projects can have different permission schemes to suit their needs.