Welcome to CPF, lagman!
3.6V refers to the nominal voltage of a Li-ion cell. Some (like IMR aka Li-Mn chemistry) have lower internal impedance than the common Li-Co, so will -on average- output a somewhat higher voltage during discharge. Hence they're often labeled with 3.7V (or even 3.8V?) as nominal voltage.
Both these chemistries are charged up to 4.2V (so Li-Co and Li-Mn/IMR cells use the same setting). That is sort of an arbitrary value as well, which offers a good compromise between safety, utilizing capacity, and longevity of cells. 4.1V would work too.
3.2 or 3.3V nominal voltage (3.6V charge voltage) is for LiFePO4 cells. And these days there are some cells that safely charge to a bit higher voltage (4.35V), hence a 3rd voltage setting.