Royal Canin splits into three: the everyday Feline Health and Care range, the breed-specific recipes, and the veterinary diets your vet prescribes. They're sold the same way but they're not interchangeable, so it's worth knowing which you're looking at.
What the numbers mean
Indoor 27, Sensible 33, Regular Fit 32. The number is the protein percentage, which makes comparing them straightforward once you know. Indoor 27 is lower protein and lower calorie because a cat that never goes out burns less. Sensible 33 is higher, aimed at cats with a delicate stomach. [Confirm with Royal Canin before publishing]
It's a useful shorthand and almost nobody explains it, so if you've been picking between them by the picture on the front, that's the difference.
The functional recipes
Most of the everyday range is built around a specific job rather than a flavour: Indoor for cats that don't go out, Sterilised for neutered cats, Hairball Care for long coats, Appetite Control for cats that pester constantly, Hair & Skin, Dental Care, Digestive Care and Urinary Care.
These are ordinary complete foods formulated with a purpose, not treatments. Urinary Care is not the same thing as the veterinary Urinary SO diet, and if your vet has diagnosed a urinary problem you need the prescription one.
Breed-specific food, honestly
Royal Canin is better known for this than anyone. British Shorthair, Maine Coon, Persian and the rest each get their own recipe, and the main real difference is the kibble itself: shape and size designed for how that breed picks food up and chews it. Maine Coon kibble is large, Persian is almond-shaped for a flat face.
That's genuinely useful for a Persian. For most cats it isn't necessary. If you've a British Shorthair and it eats Sensible 33 happily, there's no need to pay more for the breed version. Buy breed-specific if your cat struggles with ordinary kibble or you want the formulation, not because the box has the right cat on it.
Very old cats
The Ageing range goes to 12+ and 15+, which is further than most brands bother with. If you've a cat in its late teens, that's worth knowing about, because the phosphorus and protein levels are adjusted for kidneys that have been working a long time.
The veterinary range
Renal, Urinary SO, Gastrointestinal, Sensitivity Control and the rest are for diagnosed conditions. They work by restricting things a healthy cat needs, so they aren't an upgrade and some are unsuitable for a well cat.
If your vet named one, buy that exact product, including the same variant. Renal in gravy and Renal in jelly are formulated the same, but Urinary SO and Urinary Care are not the same food at all.
Wet as well as dry
Royal Canin does most recipes in both, which makes it easier than most brands to feed wet and dry from the same range. Worth doing if your cat is a poor drinker. Count both towards the daily total rather than adding one on top of the other, and if you're buying the large pouch boxes, make sure your cat likes it in a small pack first.