The first digit is the class
Ten classes cover everything: 1 elements, 2 sulfides and sulfosalts, 3 halides, 4 oxides and hydroxides, 5 carbonates and nitrates, 6 borates, 7 sulfates, 8 phosphates, 9 silicates, 10 organic compounds.
Silicates are the largest class by a wide margin, which is unsurprising given the crust is mostly silicon and oxygen.
The letters narrow the structure
The letter pair after the class digit gives division and subdivision — for silicates this tracks how the tetrahedra connect, from isolated units through chains and sheets to full frameworks.
The final number identifies the group within that subdivision. Two minerals sharing the first two segments are structurally related, not merely alphabetical neighbours.
Why a collector should care
Structurally related minerals tend to occur together, weather similarly and respond to the same cleaning methods. A shared prefix is a genuine prediction about a specimen.
It is also the fastest way to find the species you should be comparing against when an identification is uncertain.
A worked example
Take 9.AG.20. The leading 9 places it among the silicates, immediately telling you the structure is built from silicon-oxygen tetrahedra. The letters narrow it to a division and subdivision within that class, and the trailing 20 identifies the specific group.
Two species sharing 9.AG are close structural relatives. Two sharing only the 9 have little in common beyond being silicates, which covers roughly a fifth of all known species. The precision you get depends entirely on how many segments match.