What are the implications and semantics for choosing a mode in Unison's distributed package?

I searched through the documentation but may have missed where unique type Mode = Parallel | Sequential is discussed. In particular I’m trying to use it in Seq.fromList, which says:

Seq.fromList : Mode -> [a] -> Seq k a

…

All branches in the result tree will have the provided Mode

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Is this a means of controlling nested parallelism, as seems to be indicated, or is it something else?

>Solution :

Yes, that’s right. Certain operations like reduce will examine the Mode and use that to decide whether to use parallelism when processing that node in the tree.

Also see this article section discussing controlling the granularity of parallelism