darcs and pijul have an internal model that tells it where patches can go; it does not suggest patches can go pretty much everywhere (in fact; at any given time it probably only has exactly one idea of which order the patches go in). Once those patches have dependencies, it enforces their order; but e.g. groups of dependent patches can still be transplanted around.

The Darcs wiki has some cool graphics around cherry-picking merges that might answer your question better than a paragraph of prose can: http://darcs.net/Using/Model#merging-with-cherry-picking