You use the Slice Tool to define your slices.
It’s not an outside script as such, it’s using Aseprite’s CLI, a .bat file is just a way to save the command for later reuse instead of typing it every time. The only reason it’s necessary to use the CLI is because the GUI export dialogue is missing many features Aseprite actually has - the CLI docs are actually linked right next to Exporting, because they fill out Aseprite’s exporting feature set.
Hopefully after 1.3 is done, we might see more of these features added to the GUI.