In most cases, way members of a boundary relations are also members of boundary relations with a lower admin_level.
If my memory serves me right, each such way member is supposed to be tagged with the lowest admin_level of all the boundary relations it is a member of.
That also seems to be the case in a random area in Europe where 93% of such members have an admin_level tag.
However I am no longer able unable to find this tagging practice in the wiki.
I think it also has to do with the ability to render correctly the lines, as is documented .
Just checked the Italian ‘practice’ since most of these boundaries were mapped in the early OSM days. I’d consider repeating the admin levels on the boundary segments when already in the boundary relation a duplication but here all have them. Of course name:left/name:right, but since all are level 8, can’t see what is ‘lowest’. If lowest means that 8 is lower than 4, then when looking at a boundary here between 2 regions of level 4 also involved in boundaries for levels 6 and 8, there’s admin_level 4 on it i.e. highest in ranking.
I mean “lower” in the sense that admin_level=2has a lower integer value than admin_level=4.
This allows the international border, which is also a sub-national boundary to be easily identified as an international border line and be rendered accordingly.
BTW, think to have read a while back that name:left/name:right on boundaries also was no longer needed but since everything is so static here, not really delved into it.