The term comes from state law, not from marketing. It is the legal name for a boarding run.
The definition
California Health and Safety Code section 122380 defines a permanent or fixed enclosure as a structure, including an exercise run, kennel or room, that provides for effective separation of a pet from its waste products. A crate or cage is a different thing, a temporary enclosure. The separate entry on temporary enclosure covers it.
What the law requires of it
The enclosure must be structurally sound, cleanable, and big enough for a pet to turn around, stand, sit and lie down comfortably, with heating, cooling or shade as needed. For a cat, a fixed enclosure must also provide an elevated platform suited to the cat's size. A facility must tell owners in writing the square footage of the enclosure.
How Happy Paws fits
The 100 indoor/outdoor runs are fixed enclosures. Sizes are published: Economy about 16 square feet inside and 48 outside, Business Class about 30 and 60, First Class about 48 and 72. Cats have individual four-level towers. Dogs from one household may share by the owner's choice, since the law allows more than one pet in an enclosure with the owner's consent.
Why it matters to an owner
Knowing the term helps you read a kennel's notice. When a facility gives you the enclosure type and size in writing, it is following the same list. See indoor/outdoor run and California's pet boarding law in plain English.
Reading a kennel's notice
A facility that follows the law will hand you a written notice with the enclosure's square footage, the staffed hours, observation practices and the daily activity schedule. Compare those numbers with what you see on the tour. A dog should be able to stand, turn and lie down with room to spare, and the surface should be easy to clean. If a number is missing, ask for it. The kennel's published sizes are on the kennel sizes compared page.