Each symbol predominantly represented a single English sound (including affricates and diphthongs), but there were complications due to the desire to avoid making the ITA needlessly different from standard English spelling (which would make the transition from the ITA to standard spelling more difficult), and in order to neutrally represent several English pronunciations or dialects.
There were also several different ways of writing unstressed [ɪ]/[i] and consonants palatalized to [tʃ], [dʒ], [ʃ], [ʒ] by suffixes.
There are two distinct ligatures for the voiced and unvoiced "th" sounds in English, and a special merged letter for "ng" resembling ŋ with a loop.
A special typeface was created for the ITA, whose characters were all lower case (its letter forms were based on Didone types such as Monotype Modern and Century Schoolbook).
Where capital letters are used in standard spelling, the ITA simply used larger versions of the same lower-case characters.
So a new character, the "half-hook a", was devised, to avoid the necessity of producing separate instructional materials for speakers of different accents.