The Underworld is functionally an 'afterlife' of sorts. Much like FF7's lifestream, it's where souls go to be reconstituted after their corporeal forms die. What happens to those memories that are cleansed from the souls hasn't really been addressed. It could result in something akin to FF9's Memoria with a 'planetary' memory, as was previously suggested, or they could simply disappear altogether. In fairness, the revelation on Elpis that the aetheric signature of a past event can sometimes be read from the land itself does seem to suggest the concept of a 'planetary memory'.

A running philosophical point in this story is the idea that 'who you are' is very much linked to your memories. That was very much central to Elidibus, Gaia, and Amon's stories. It essentially underpins the job system as well. The soul crystals carry the memories of the myriad deeds of previous practitioners of the art. As an example, when you pick up the Dark Knight soul crystal, in a way, you inherit Fray as well.