Task statuses allow you to manage your project progress in more detail.   In addition to the overall project statuses, you can set a detailed, independent status on each of the project's tasks.

Tasks status's are setup in Administration -> Settings -> Project Settings.  

Enter additional statuses to Custom Task Statuses.  These will be added to the default status list (Cancelled, Completed, In Progress, On Hold, Waiting To Start).

Task status's can then be applied on the task's status dropdown in projects and production job Overview tab.

Select the required task status from the dropdown:

Selecting the default Completed status will mark the task as completed for the completed tasks invoicing function and prevent additional timesheets to be added to the task.