You will see the objective field in the Task window in the case that you have a Client. Objectives are necessary to complete a Project, for more information on objectives click here
You can assign many Tasks to an individual objective. All Tasks must be completed for the objective to be complete.
This is an example of where you can choose an objective if it has been created in the Client feature before.
Note: the Client feature and objective are only available for Premium Projects.
To see all benefits of upgrading your Project to Premium, click here.