in the cases of Valentines Lace, Lucky Dust, and Spring Blossoms Disruptors, it is random unless the pet in question has a trait in the slot where the disruptor will be placing the trait, which means it has to use the trait from that specific slot
(this is also how the other rtds work, but they're harder to predict since they have more than one trait in their trait pools. Unless you have specifically set up a pet to receive a specific trait by breeding so all other trait slots that disruptor could fill are carries or have res. trait genetics from other res trait sets)

(more indepth breakdown on rtds here