This property defines the Unicodes which belongs to Cyrillic block
Characters like Ѐ,Ё,Ђ, etc. are listed under Cyrillic Block.
256 Characters are present in Cyrillic Block starting with the Unicode point U+0400 that represents the character Ѐ.
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.