RMK has default values for used flash area, check out the default config for your microcontroller. If you're using Rust API, it's recommended to use the default values.
Storage feature is used by saving keymap edits to internal flash.
If you're using the keyboard.toml, you can set the storage using the following config:
You can also edit storage_config field in RmkConfig if you're using Rust API:
When manually setting the storage area, you have to ensure that you have enough flash space for storage feature. If there is not enough space, passing None is acceptable.