See wikipedia for explanation of the concept.
Currently tagging motorway interchanges in OSM is not trivial nor does it really represent the interchange. On a motorway interchange it's often the case that a motorway_link splits of (at a motorway_junction) the main motorway but then the motorway_link splits again (again at a motorway_junction) into two motorway_links. Now how would a router know at the second motorway_junction which road splits off and which road "goes on"?
Often motorway interchanges are named. This can also be valuable information for routing software, e.g.: "At motorway interchange Watergraafsmeer go to the motorway A1, direction Diemen" instead of "Take the next junction".
In order to achieve a more "semantically correct" tagging will have to change a bit.
The point at which a motorway spits.
|name||name||This is often the direction that is first named on the junction-boards|
|ref||ref||The "number" of the exit|
All the roads on a motorway interchange have a direction. With respect to completeness even the motorways that go "straight on" are captured in a direction relation. On a motorway_junction the motorway splits into at least two roads. Each road has (at least) one direction. A direction relation contains all the ways that go in that direction, this does not imply that the roads have to be connected (but they often are connected at some point).
As a result a way can be in multiple direction-relations. The router can then decide which direction is named, or maybe all are named, e.g.: "At interchange Zaamdam continue following the A8 in the direction of Alkmaar and Beverwijk".
|name||name||The direction of the roads in this relation.|
All the direction relations together make up the motorway interchange. So these are added as members of the interchange relation. All the motorway_junction nodes that are "part" of the interchange relation are also added to the relation. Often a motorway interchange has a name so that can also be added here. Of course other tags like alt_name=* etc can also be added.
|junction||motorway_interchange||This is often the direction that is first named on the junction-boards|
|name||name||Name of the interchange|
|direction-relation||At least two||A direction relation on this motorway interchange|
|motorway_junction-node||At least two||A motorway_junction node that is part of this motorway interchange|
Relation 1076017 is tagged as an example to this scheme.