The features referred to in the key refer to dialects marked by their pronunciation of certain words, especially the word for 'what?', which differs radically in these dialects. In some dialects it is "kai" ( Kajkavci or "Kaikavian"), in others it is "cha" ( Cakavci or "Chakavian"), etc. The top three mentioned (in the key chart) are themselves grouped into a general "Shtokavian" complex, ( shto is the word for 'what?' in those dialects, and in other Slavic languages, e.g. in Russian, as well) while the next group is "mixed".
These features do not coincide in any way with the usual divisions into Serbian and Croatian, i.e. they can not be used to determine whether someone is a Croat, a Serb, or a Bosnian. (In fact, during the conflicts following the collapse of Yugoslavia, people who were captured and/or interrogated by one hostile group or another were not identified by their speech, but by their names which would identify them as Catholic, Orthodox, or Muslim.)
Note the artificial straight-as-an-arrow boundary between what is indicated to be "Macedonian" (supposedly a separate language, but closer to Bulgarian than anything else) south of the Serbian area. Note also that Kosovo, known even in 1954 to be 90% Albanian-speaking, is given as a totally Ekavcian area, totally congruent with eastern "Serbian" dialects. The presence of large numbers of Hungarians in the area around Novi Sad is also ignored; other ethnic languages (Greek, Rumanian [Vlach], German etc.) are also ignored. The map, then, which tries to be non-political when it comes to the Serbo-Croatian dialects, gets political when it gets to certain borders, such as the Italian or Austrian border, where suddenly, language habits change!
Note: I added language names such as "German", "Hungarian" etc. to clarify what other languages are on the borders of this area.
For a really large-scale map, go to this map
And for a political map of the area, go to this map.
Here is a better political/ethnic map of the region, from the National Geographic and its key.
See also this source, which documents the disintegration of a unified Serbo-Croatian into 'separate' languages:
Greenberg, Robert D. Language and Identity in the Balkans. Serbo-Croatian and its Disintegration Oxford: Oxford University Press (2004)