What does edge_norm mean in R-GCN?

In
https://docs.dgl.ai/en/latest/tutorials/models/1_gnn/4_rgcn.html?highlight=edge_norm
there is “edge_norm”, but I can’t find out how to define this and where this came from.

Node_norm constant definition is found at original R-GCN paper, but edge_norm is not.
What is edge_norm? Where this came from?