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There are two differences:

1. Items in single action lists are styled in blue text (by default), whereas the first item in a parallel project is purple and the rest are black.

2. Items in single action lists and parallel projects will all show if you filter to display "available" actions. On the other hand, if you filter to show "next actions", then all the items of a single action lists items will still show, but only the first item of a parallel project will show.

It's a bit of a compromise design. You can scroll back to the forum threads from 2+ years ago if you'd like to read some of the debates that raged about the definition of "next action" in a parallel project.