It is a weighted binary cross entropy loss + label non-co-occurrence loss. weights and uncorrelated pairs are calculated beforehand and passed to the loss function.

This is the loss function.
First compute the set of uncorrelated pairs (as per the training data); Su = {i, j | M(i, j) = 0, i < j, 1 ≤ i, j ≤ q}. For each (i, j) ∈ Su, we sum the model-based conditional co-occurrence probabilities Pˆ(j|i)
and Pˆ(i|j) here