Originally Posted by RocknBlogger
So when I enter a task do I need to click the arrows to specify? Or is there any automation for this feature?
No, projects default to one or the other, see the Projects & Action groups section of the Data pane of the OmniFocus preferences. You can change it on a project-by-project basis by clicking on those arrows or by using the inspector. You can also mix and match as needed:
In this project, some items need to be done in a specific order, and others do not. The overall project is sequential, but sections of it are parallel. "Spread dropcloth" will not be offered as an available action until all of the tasks in "Buy paint and tools" have been marked complete (including the "Buy paint and tools" container task).