Alloy numbering systems as per ASTM vary from one alloy group to the next. To avoid confusion, the UNS (short for "Unified Numbering System") for Metals and Alloys was developed.
The UNS number is not a specification, because it does not refer to the method of manufacturing in which the material is supplied.

In the UNS number scheme each metal is designated by a letter followed by five numbers. It is a composition-based system of commercial materials and does not guarantee any performance specifications or exact composition with impurity limits.

An outline of the UNS designations is as follows:

UNS Series Metal
A00001 to A99999 Aluminum and Aluminum Alloys
C00001 to C99999 Copper and Copper alloys
D00001 to D99999 Specified Mechanical Property Steels
E00001 to E99999 Rare Earth and rare-earth-like Metals and Alloys
F00001 to F99999 Cast Irons
G00001 to G99999 AISI and SAE carbon and alloy steels (except tool steels)
H00001 to H99999 AISI and SAE H-Steels
J00001 to J99999 Cast Steels (except Tool steels)
K00001 to K99999 Miscellaneous Steels and Ferrous Alloys
L00001 to L99999 Low-melting Metals and Alloys
M00001 to M99999 Miscellaneous Nonferrous Metals and Alloys
N00001 to N99999 Nickel and Nickel Alloys
P00001 to P99999 Precious Metals and Alloys
R00001 to R99999 Reactive and Refractory Metals and Alloys
S00001 to S99999 Heat and Corrosion Resistant (Stainless) Steels
T00001 to T99999 Tool Steels, Wrought and Cast
W00001 to W99999 Welding Filler Metals
Z00001 to Z99999 Zinc and Zinc Alloys