[whpalmer4]

Let's see if I can make this more confusing :-)

There are parallel and sequential projects, and parallel and sequential action groups. Single action lists are a special class of parallel projects. Action groups are just nested actions, allowing you to break a task down into smaller components. Both projects and action groups can contain action groups of either sequencing type. For example, you could have a project that has 3 main tasks, each of which is a sequence of actions which must be done in order, but you could work on any of those 3 main tasks at any time. Or you could have a sequence of tasks that had to be done in order, but in the middle of that, have an action group with a bunch of parallel actions which can be done in any order, but they all need to be done before you can move on to the next part of the sequential chain. Parallel projects and action groups are treated by OmniFocus as if the order in which you have placed the actions is important to you, but that is merely for your convenience.

As for single action lists and ongoing responsibilities, the distinction is mainly that you don't expect to complete the "project" in any meaningful way. There's nothing stopping you from doing so, however; it is just a convention. There's nothing wrong with using a parallel project for the job if the behavior suits your workflow better. It's just a few clicks to switch it from one to the other, and no state is lost in the change, so you can freely switch between the two.