Parallel and sequential projects explained
When you open a project's detail window, you may switch between or .
Sequential projects always have only one active task at a time. This option should be used when you have to complete the first undone task on the list to start on the next one: you have to work on the tasks sequentially.
You can tell that a project is sequential on the project's task list, because only the first task in the list has a black name and a checkbox on its left side. All the other tasks have grey names and no checkbox on their left side. If the task is delegated, paused, completed or dropped, it will have a checkbox telling you that it's not actionable.
Parallel projects will show all of the tasks and their checkboxes. This is the default option, and should be used when you don't have to complete one task before another – you may work on all of the tasks in parallel.