The ability to train dogs, and to get them to do useful things. Those things might include:
Roll vs. "Easy" - Canine handling and training
Get a dog to do something for which it's been successfully trained.
Roll vs "Normal - Canine Training
Pest control (many small breeds do this instinctively, but training can help hone a ratter's skills).
Draft (pulling sleds, skiiers and the like)
Retrieving game
Difficult - Canine Training
Attack (successfully trained dogs will attack on command, and do so more effectively)
Guard
Herding
Tracking
Formidable - Canine Training
Tracking (makes characters' Tracking rolls one level easier)
Detection (explosives, drugs, other substances)
All tasks are one level easier with a dog breed that is bred for/suited to the job being trained
An extremely incomplete set of examples:
Terriers are well adapted to controlling rats and mice.
Huskies were bred as draft dogs
Bloodhounds are incredibly adept trackers.
German Shepherds and Malinois are well adapted to detection and/or attack
Collies and Sheepdogs were bred for herding
Conversely, it's a level more difficult for breeds that are unsuited to a task - trying to train a bloodhound as a draft dog, or a husky as a tracker, is going to tax a trainer's skills.
Picking appropriate breeds for specific jobs is left to the player character or referee. If you wind up with a bunch of Yorkie attack dogs, don't come crying to the author.