I think this is quite clear and a logical choice. Maybe it wasn't there at first.
In the former Soviet/Russian empire territory, some lands were settled mostly by Slavs, some have mixed populations of Slavs and non-Slavs, such as Kazakhstan and Latvia (which have a lot of ethnic Russians). Armenians and Georgians, for example, are their own ethnic groups that speak languages unrelated to Russian/Ukrainian/other Slavic languages.
(almost?) all ex-USSR would be included.
Countries with Slavic speakers (including non-native speakers)
Countries with a Slavic Official Language (including regions, municipalities etc)