IPA numbers are a legacy system of coding the symbols of the International Phonetic Alphabet. They were the organizational basis for XSAMPA and the IPA Extensions block of Unicode.
Following the Kiel Convention in 1989, most letters, diacritics and other symbols of the IPA were assigned a 3-digit numerical code, with updates through 2005. The purpose was to identify IPA symbols explicitly in an era of competing computer encodings, and thus to prevent confusion between similar characters (such as ⟨ɵ⟩ and ⟨θ⟩, ⟨ɤ⟩ and ⟨ɣ⟩, ⟨ʃ ⟩ and ⟨ʄ ⟩, ⟨ɫ⟩ and ⟨ɬ⟩ or ⟨ǁ⟩ and ⟨‖⟩) in such situations as the printing of manuscripts. The system never saw much if any use and is now defunct, having been superseded by Unicode.[1]
The semantic and graphic categories of the symbols are assigned different ranges of numbers: The 100 series are IPA consonants, the 200s retired and non-IPA consonants, the 300s vowels, the 400s diacritics, the 500s suprasegmentals, the 600s extIPA, the 700s capital letters and the 900s delimiters.[1] Some symbols have more than one code.[2]