Gaël Montreuil (asker) Apr 10, 2019:

thanks a lot for the explanation, makes much more sense now. These tiny typos that make the text less understandable are annoying!

Tony M Apr 10, 2019:

@ Asker Oh golly, yes, I've been using these things for donkey's years! The (clever) idea is that the square nuts (in this case, with an M6 thread) are attached within a springy 'cage', which is then simply 'snapped' (= 'clipped') into square holes in the rack uprights, allowing just enough movement of the nut to be able to align the threads of the rack-mount screws. A special tool is used to remove them more easily. When the system works correctly, it is ingenious; sometimes, it can be a bit fiddly!

By the way, I reckon it should say "... snap an M6..." (because in EN we'd say that as 'em-six'), not 'and', which would make no sense here.