Certain breeds have special styles for their grooming including haircuts and trims. Other dog owners don't want to follow the book on the breed they own; rather, they want the dog shaved down or styled to their choosing. Oreo with the puppy cut above (this one is 3/4") with a round face is a popular one for Shih Tzus that are family pets and not show dogs. The puppy cut means the body is all the same length and can be done for a dog of any age, and any breed that grows hair rather than shedding it.
As a mobile groomer, when I meet a new customer and their pet, I always ask how THEY want to dog to look. If the say, "Just make her cute." - then I use my creativity. But it is the owner that has to look at the dog every day, so they should choose the style. The owner of Cookie below likes a modified skirt on him so she doesn't have to see his private parts. He's not fat - that's two inches of skirt hanging from his belly!
Some dogs like the Cavalier below just need a de-shedding bath with furminating shampoo and conditioner. Then a good brushing when dry. Other Cavis I shave down because they play outside often and get very dirty.
I shave a dog or cat with a #5 for a little fluff, a #7 for a typical shave or even a #10 for pets with skin issues such as fleas, allergy scabs or if there are family members allergic to the pet. The black Pomeranian below had a #5 shave down due to skin issues.
Groomers and owners should discuss the final result desired before grooming begins. Pictures and photos of the pet will help clarify the idea of the perfect groom.