By project we mean a set of client jobs. Client jobs cannot be created without creating a project first.
Each project has only one client. Though you can create a lot of projects for one client.
You can imagine a project as a folder and client jobs as files, stored in this folder.

I.e.: To add a new client job, you need to create a client record, if the client is new, then create a project for the client and finally add the job to this project.