There is a formulation in EGNN.
Each edge feature $\mathbf{e}_{i j}=\left\{e_{i j d}\right\}_{d=1}^{2} \in[0,1]^{2}$ is a 2-dimensional vector representing the (normalized) strengths of the intra- and inter-class relations of the two connected nodes. This allows to separately exploit the intra-cluster similarity and the inter-cluster dissimilairity.
Edge features are initialized by edge labels as follows: $$ \mathbf{e}_{i j}^{0}=\left\{\begin{array}{cc}{[1 || 0],} & {\text { if } y_{i j}=1 \text { and } i, j \leq N \times K} \\ {[0| | 1],} & {\text { if } y_{i j}=0 \text { and } i, j \leq N \times K} \\ {[0.5| | 0.5],} & {\text { otherwise }}\end{array}\right. $$ where || is the concatenation operation.
It is about edge features. But I don't know the meaning of the concatenation operation. I don't know the '||' in this formulation. Also, It is hard to search it on google. So I ask help to the community.