U+1D00 Latin Letter Small Capital A

The Wikipedia has the following information about this codepoint:

The International Phonetic Alphabet (IPA) possesses a variety of obsolete and nonstandard symbols. Throughout the history of the IPA, characters representing phonetic values have been modified or completely replaced. An example is ⟨ɷ⟩ for standard [ʊ]. Several symbols indicating secondary articulation have been dropped altogether, with the idea that they should be indicated with diacritics: ʮ for z̩ʷ is one. In addition, the rare voiceless implosive series ƥ ƭ ƭ̢ ƈ ƙ ʠ has been dropped.

Other characters have been added in for specific phonemes which do not possess a specific symbol in the IPA. Those studying modern Chinese phonology have used ⟨ɿ⟩ to represent the sound of -i in Pinyin hanzi which has been variously described as [ɨ], [ɹ̩], [z̩] or [ɯ]. (See the sections Vowels and Syllabic consonants of the article Standard Chinese phonology.)

There are also unsupported symbols from local traditions that find their way into publications that otherwise use the standard IPA. This is especially common with affricates such as ƛ, and many Americanist symbols.

While the IPA does not itself have a set of capital letters (the ones that look like capitals are actually small capitals), many languages have adopted symbols from the IPA as part of their orthographies, and in such cases they have invented capital variants of these. This is especially common in Africa. An example is Kabiyé of northern Togo, which has Ɔ Ɛ Ŋ Ɣ. Other pseudo-IPA capitals supported by Unicode are Ɓ/Ƃ Ƈ Ɗ/Ƌ Ə/Ǝ Ɠ Ħ Ɯ Ɲ Ɵ Ʃ (capital ʃ) Ʈ Ʊ Ʋ Ʒ. (See Case variants of IPA letters.)

Capital letters are also used as cover symbols in phonotactic descriptions: C=Consonant, V=Vowel, etc.

This list does not include commonplace extensions of the IPA, such as doubling a symbol for a greater degree of a feature ([aːː] extra-long [a], [ˈˈa] extra stress, [kʰʰ] strongly aspirated [k], and [a˞˞] extra-rhotic [a]), nor superscripting for a lesser degree of a feature ([ᵑɡ] slightly prenasalized [ɡ], [ᵗs] slightly affricated [s], and [ᵊ] epenthetic schwa). The asterisk, as in [k*] for the fortis stop of Korean, is the convention the IPA uses when it has no symbol for a phone or feature.

For symbols and values which were discarded by 1932, see History of the International Phonetic Alphabet.