CURRENTLY NOT IMPLEMENTED

It is an in-game notepad.

It is not a magic tool.
Basically used for marking areas in the game where you need to do something.
It is called Reroll Helper because originally it was to help those who reroll a lot (for example, sellers) to follow a set of markings.
However, you can use it to mark stuff in Open World as well. Some use it to remind them of puzzle solutions or how to solve events like Inazuma Treasure Hunt in 2.0.
Very helpful for people with multiple accounts.