Pointed by Ben Asher, and Endorsed by Maimonides

A Hebrew Bible manuscript carries two layers. There are the consonants, which are what a Torah scroll contains and which had been stable for centuries. Then there are the vowel points, the accents that govern chanting, and the masoretic notes in the margins that count words and flag oddities so that a copyist cannot drift. That second layer was the work of the masoretes of Tiberias, and its final and most authoritative form is associated with one family, the ben Ashers.

The Aleppo Codex is the only surviving manuscript in which that apparatus was supplied by Aaron ben Moses ben Asher himself. The consonants were written by another scribe, Shlomo ben Buya'a; ben Asher pointed, accented and annotated them. Nothing else in existence is that close to the source.

Its authority was settled in the twelfth century. Maimonides, working in Egypt, examined the codex and adopted it as the standard for his rules on how a Torah scroll is to be written, down to the layout of the poetic sections. Once he had done that, every later halakhic authority inherited the judgment. For roughly eight hundred years afterwards the manuscript was treated as the yardstick against which other Bibles were measured, which is a large part of why the Jewish community of Aleppo guarded it so closely and showed it to so few.