Expected parameters in beamer@makecaption

I am trying to understand the semantics of the macro beamer@makecaptionin Beamer in order to modify the code in List of Figures + Beamer? (followup)

It looks like it expects 4 parameters, where #1 is table or figure, #4 is the optional parameter in caption and #2 and #3 are the main parameter in caption after a bit of adjustment, like adding an ignorespaces or similar command. But what is the difference between #2 and #3 and why could one of them be empty, as the mentioned code suggests.