this function was named StorAge Selection function (SAS).
In many physical cases, \(t_{in}\) and/or \(t_{ex}\) are not random variables and therefore, it could be improper to talk about probabilities. However, using this nomenclature is still efficient and bringing immediate understanding of the properties of the functions. So we will maintain it also in those cases, instead of talking about fractions. 

Determination of \(R\left(t,t'\right)\)