This property defines the Unicodes which belongs to Phonetic Extensions block
Characters like ᴀ,ᴁ,ᴂ, etc. are listed under Phonetic Extensions Block.
128 Characters are present in Phonetic Extensions Block starting with the Unicode point U+1D00 that represents the character ᴀ.
Currently, as per latest version 15.0.0, Unicode supports 328 number of Block. Here are a few examples - Latin Extended E,Adlam,Aegean Numbers, etc.