The project elements they are linked to.
Once access to the project is granted, clients and contributors can only see tasks, comments, and documents that are linked to them.
To learn how to add and manage contacts related to an ongoing project, refer to this article.
Useful Tips : Contacts added to the environment will only have access once you grant it to them. To do this, check the "Access to the platform" option. You can revoke their access at any time by unchecking the boxes in the project settings.