In neo4j to relate 2 nodes you can make a simple relationship or make a RelationshipEntity, as shown in: relationship-entity.
Using a simple relationship you can add properties such as ... CREATE (Hugo)-[:ACTED_IN {roles:['Agent Smith']}]->(TheMatrix) ...
my question is: when is it appropriate to use RelationshipEntity ?, would it be in a relation n: m of a relational database?
Why in relationship-entity utilize relationshipEntities and not a simple relationship?
From documentation Relationship-entity
To access the full data model of graph relationships, POJOs can also be annotated with @RelationshipEntity, making them relationship entities. Just as node entities represent nodes in the graph, relationship entities represent relationships. Such POJOs allow you to access and manage properties on the underlying relationships in the graph.
It used depends of what you need to achieve. You can used it to add more details about the relationship if you need. May be add a price in buying command.
CREATE (c:Customer {name:'User Test'})-[b:BUY {quantity:12}]->(i:Item {name:'Orange', price:15.0}) RETURN c,i
In my case I have used it only during cypher learning stage