RelationshipType is similar to the concept of "association" in the UML, or "relationship" in information modeling. In InfoGrid so far, a RelationshipType is always binary, i.e. its instances (Relationships) each connect exactly two MeshObjects.
A RelationshipType may either be directed or undirected. If it is directed, the source and the destination MeshObject can be distinguished by a distinct RoleType. If it is undirected, the two related MeshObjects play the same RoleType.
For example, the RelationshipType isParentOf is directed, as there is clearly a semantic difference in whether "A is a parent of B", or "B is a parent of A". On the other hand, the RelationshipType isRelativeOf may be undirected, as there is no semantic difference between "A is a relative of B", and "B is a relative of A".