八識規矩補註 1.32

1.32

此言第六識遇善境時，與善心所相應，遇不善、無記境時，與不善、無記心所相應，故曰別配之。據理談情，思之可悉。

This means that when the sixth consciousness encounters a good object, it is associated with good caittas; when it encounters an unwholesome or avyākṛta object, it is associated with unwholesome or avyākṛta caittas; hence it is said to be 'differentiated according to occasion.' When one considers the facts on the basis of principle, this can be understood through reflection.