I was wondering the same. My impression was that "Half truth" was going to be for statements that (while intended to mislead) were logically true without heavily twisting words, or using obscure meanings.
That is, "Luke, we lost your father to a battle with the Sith," or "your father, a promising Jedi, was defeated by Darth Vader" would both be half truths - it is clear that they are not lies. Things like "he was alive when I last saw him [and shot him in the gut]" fall under Half Truth as well.
I thought "Metaphorical Truth" was going to be for statements like "Darth Vader betrayed and murdered your father". For the "murder" part of that statement in particular to be true, we're dependent on a metaphorical, or unusual, reading of the words (in this case, it seems like an outright lie if you don't know (amongst other things) that the Jedi regard the Sith as akin to the walking dead).