That's could be a newbie question.
There is an abstract function $ \mathbf {h(X_i,X_j)}$ what does mean
for some function h mapping $$\mathbb R^q \times \mathbb R^q \to \mathbb R^p$$ where the q - dimensional row vectors $\mathbf X_i$ and $\mathbf X_j$ are the nodal covariate vectors for the i-th and j-th individuals.
What happens with vectors X?
Thanks for patience =)