Sure, because that line forms the boundary. It wouldn’t be right to have a little gap of territory which isn’t within any municipal boundary. The exception to that rule, of course, is where a gap exists in the real world.
I make up a border out of some nodes.
I put these nodes into relation A which is the border of MunicipalityA
Additionally some of these nodes belong to the border of MunicipalityB (Relation B)
What has this got to do with ways?
^^^^^^^^^ Tag the WAY!!
To make my point clear - In my point of view it is a relation that is made up from nodes:
the yellowish line is the border, it’s actually just 9 nodes.
Or should I ‘combine’ these nodes into a WAY. And then create a relation that has just one member → the WAY. This will be the border of the northern town and then again I add this WAY to the borders of the ‘southern’ town.
Yes, borders are ways, not collections of nodes. Don’t think of a ‘way’ in OSM parlance as a road. A way in OSM is the name for any linear feature, including boundaries.
Where 2 administrative regions ‘touch’, they share a common way. Each region gets its own relation, with the shared way as a member in each of these relations. In the end, you would build up the entire boundary around an administrative region this way.