If you are wanting to organise your dock with separators, you can run the command below in terminal to create a spacer.
>> defaults write com.apple.dock persistent-apps -array-add '{ "tile-type" = "spacer-tile"; }'
killall Dock
A line separator can also be “installed” but this is creates vertical lines which is only suitable if the dock is horizontal along the bottom.
Ref:
- https://apple.stackexchange.com/questions/168327/create-a-line-separation-for-finder-in-the-dock-osx-10-10