text: The wiki page for milestones is at https://wiki.openstreetmap.org/wiki/Tag:highway%3Dmilestone; the taginfo link on the page shows that around 48% of all milestones have a ref= tag. I don’t have any documentation on correct usage of the ref tag, however in practice it is the ref of the road on which the milestone is located. Examples can be found just a bit further south, along the DK11, or along the A2 and A4 motorways.