What is the difference between Sahel, Sudo-Sahel, West Africa semi-arid tropics?

Does anyone happen to know? Any geographers out there? These terms are thrown around in literature without any definition. As I understand it, the Sahel is a narrow band of land stretching across the African continent between the Sahara desert and sub-Saharan savannah. I assume there is no distinctive boundary on either side but that it is a somewhat arbitrary transition gradient. I also assume that “West Africa Sahel” therefore refers to the portion of the Sahel in “West Africa”, as defined on Wikipedia. The FAO defined “West Africa semi-arid tropics” as “those areas where rainfall exceeds potential evapotranspiration for two to seven months annually. This area encompasses all of Senegal, the Gambia, Burkina Faso and Cape Verde, major southern portions of Mauritania, Mali and the Niger and the northern portions of Cóte d’lvoire, Ghana, Togo, Benin and Nigeria. ” I think I read that “The Sahel” is divided in two, the sudo-sahel and…? Is there a map that overlays all of these different classifications? And what are the driving forces behind so many different groupings in this region?