Fonts are kind of dumb. There's usually nothing actually weird going on. Ligatures are simple character substitutions. What the LIGSPACE is, most likely, is a spacing character added by the designer to align the characters in the ligature correctly. So, the LIGSPACE is just a character like any other and is nothing special. For example, when you hit a space bar, you are inserting a character which doesn't render anything because the designer didn't draw anything there. There's no extra logic.