As far as I know, currently it's only used for move detection in the diff viewer:
Hide Unordered Moves - available when "Track Moved Nodes" is on, ensures that reordered nodes in a collection defined as "unordered" in the structure aspect are not highlighted as a change.
I would also like to know. It is mentioned in the documentation about light patterns (at the very end) but I also couldn't find a way to create an unordered link.
EDIT: I found a way: You can do it in the inspector! (I should have guessed that.)
As far as I know, currently it's only used for move detection in the diff viewer:
– https://www.jetbrains.com/help/mps/differences-viewer.html