How does the game determine which Random Scripted Encounter occurs?

[fafaDylan]

As the question above says, I am hoping somebody knowledgeable can explain how the game rolls the Random Scripted Encounters it throws at you. Specifically, what are gamedata files involved, and what mechanism/variable inside the files determines the encounter. For example, if I wanted to add a scripted encounter while roaming Neketaka or Dunnage or when sailing, or if I wanted to adjust the weight/frequency of event occurrence, what files should I modify? Thank you in advance.