This property defines the Unicodes which belongs to Greek Extended block
Characters like ἀ,ἁ,ἂ, etc. are listed under Greek Extended Block.
249 Characters are present in Greek Extended Block starting with the Unicode point U+1F00 that represents the character ἀ.
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.