Assignment is your contract with your freelancer. An assignment outlines the engagement specifics, such as type of contract (one-off vs. recurrent), rate for deliverable, expected completion timing, amount of pre-approved expenses etc.
Project, on the other hand, is more for your own organization. Ultimately, the terms about your engagement with freelancer is governed by the terms stated on the assignment. You can create as many assignments within one project, or have one assignment per project - all depends on your preference and how easy it will be for you to locate the assignments in the future. It does not have any legal implication on your engagement with your freelancer