This property defines the Unicodes which belongs to Bopomofo Extended block
Characters like ㆠ,ㆡ,ㆢ, etc. are listed under Bopomofo Extended Block.
32 Characters are present in Bopomofo Extended Block starting with the Unicode point U+31A0 that represents the character ㆠ.
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.