The Persian cat is a "type" breed. While coat contributes greatly to the overall final effect, it is the body and head structure that set this breed apart.
All the major cat registries accept the breed. While there may be variations in how the varieties and colors are divided and shown, the bulk of the written standard is very much the same. A show quality Persian from any registry would be instantly recognizable for what it is.
In a nutshell, a Persian should be short and round in all areas except eyes, personality, and in the case of Himalayans and Persians, coat.
It should be a medium to large-sized cat. The body should be short and compact, as wide at the hips as at the shoulders. The body should be set low on thick, muscular legs. The tail should be short.
The head should be round, large and set on a short, muscular neck. The eyes should be round and set wide apart, with a short, snub nose placed high on the face. The ears should be small, widely spaced and rounded on the tips.
Tophead is the top of the skull. It should be smooth, rounded and with good width between the ears.
This is the forehead of the cat. It should be rounded, high and lead smoothly to the tophead. This contributes to the impression of a round head.
This is the "dent" above the nose. It is where the back of the nose meets the skull. It should be deep and centered between the eyes.
Set: Lots of space between the ears, across the top of the skull.
Size: Ears are small and contribute to the overall roundness of the skull.
Shape: Ears are rounded at the top and tipped forward, again contributing to the impression of roundness.
Degrees of Extremeness
The term "extremeness" generally refers to how short and high-placed the cat's nose is. The standard calls for a short, snub nose with the break centered between the eyes. This leaves lots of room for interpretation. The photos below demonstrate various degrees of extremeness. While all four cats could easily be described as having short, snub noses and breaks between the eyes, only the top three would likely be competitive in the show ring.
Refinement does NOT mean small or fine-boned. It is the "total package". A cat can be technically a good representative of the standard, but may not be a show cat. When reading the standard and evaluating an individual cat against any other breed, many Persians will seemingly fit the standard and could be called "show quality". However, refinement is more than just a short nose, high break and/or lots of coat.
The pair of photos below demonstrates this difference. They are both 8-month-old, copper-eyed white Persians. Both have short, snub noses, large round eyes, deep breaks between the eyes. Both are heavy boned, cobby and low on the leg. However, when compared with another member of the breed, one is obviously much more refined in the standard.