The GCCF, like all other bona fide registration bodies, has an Official Standard of Points for each breed which it recognises. The Persian SOP is overseen by the Persian BAC, and any amendments are agreed by the BAC, and then passed to Council for approval.
N.B. There is also a Preface to the SOP, which is a Standard List of Withholding Faults - these apply to all breeds.
The first breed standard (then called a points of excellence list) was issued in 1889 by cat show promoter Harrison Weir. He stated that the Persian differed from the Angora in the tail being longer, hair more full and coarse at the end and head larger, with less pointed ears. Not all cat fanciers agreed with the distinction of the two types, and in the 1903 work The Book of the Cat, Frances Simpson states that "the distinctions, apparently with hardly any difference, between Angoras and Persians are of so fine a nature that I must be pardoned if I ignore the class of cat commonly called Angora". Dorothy Bevill Champion laid out the difference between the two types in the 1909 Everybody's Cat Book
"Our pedigree imported long-hairs of to-day are undoubtedly a cross of the Angora and Persian; the latter possesses a rounder head than the former, also the coat is of quite a different quality. The coat of the Persian consists of a woolly under coat and a long, hairy outer coat. In summer it loses all the thick underwool, and only the long hair remains. The hair is also somewhat shorter on the shoulders and upper part of the hind legs.
Now, the Angora has a very different coat, consisting of long, soft hair, hanging in locks, inclining to a slight curl or wave on the under parts of the body. The hair is also much longer on the shoulders and hind legs than the Persian, this being a great improvement; but the Angora fails to the Persian in head, the former having a more wedge-shaped head, whereas that of the modern Persian excels in roundness.
Of course. Angoras and Persians have been constantly crossed, with a decided improvement to each breed; but the long-haired cat of to-day is decidedly more Persian-bred than Angora.
Champion lamented the lack of distinction among various long-haired types by English fanciers, who in 1887, decided to group them under the umbrella term "Long-haired Cats".
You can download the current Persian SOP from the link above.