What is this?

Persistent Data (undertale.ini)

Things that UNDERTALE remembers throughout multiple playthroughs

General
Omega Flowey & Other

SAVE Data (file0)

This is what gets SAVED at SAVE points

General
Stats
Inventory
Cellphone
Ruins
Snowdin
Waterfall
Hotland
Other
FUN Values
Value Effect
2-39 Wrong number
40-45 Sans' call
46-50 Alphys' call
56-57 Nightmare mode
61-63 Gaster followers
65 Sound test room
66 Fake hallway
80-89 Clam girl
90-100 Goner kid
Advanced

PLEASE only proceed if you know what you are doing!

Each flag has a mouseover explanation.

More info