G-code configuration files will allow you to customize your firmware without needing to recompile it, you just need to install the firmware most similar to your printer's configuration and adapt it using standard and custom G-codes. For example, you can use it for setup a Sprite Extruder Pro or an Ender extender kit to your Ender3V2 or set your physical settings for the Neo/Max/Plus printer.

If you want to setup all your custom settings after flashing or after load defaults, you can write (with a texteditor) a small config.gco file (or another filename.gco) with content like this:

; Professional Firmware Configuration File
;=====================================================
C10                    ; Mark as a configuration file
M117 Applying configuration
;-----------------------------------------------------
C100 X0 Y0             ; Set XY Min position to 0
C101 X230 Y230 Z240    ; Set XYZ Max position
C102 X220 Y220         ; Set a bed size of 220x220
C29 L25.00 R195.00 F25.00 B195.00 N5 T50 ; Set Mesh insets, size of 5x5 and bed at 50°C
C851 S800 M0           ; Probe feedrate and disable multiple probing
C412 M0                ; Filament sensor active mode: LOW
M256 B150              ; Set Brightness 
M413 S0                ; Disable Powerloss recovery
M603 L200.00 U200.00   ; Configure Filament Change
M500                   ; Save all settings
;-----------------------------------------------------
G4 S1                  ; Wait a second
M300 P200              ; Beep
M117 Configuration Applied
;-----------------------------------------------------

A configuration G-code file should not have any motion command (G0, G1, G28, G29, etc), you can add any configuration G-code, for example you could add an M851 to set your probe offset, M92 E425 to set the extruder steps/mm or a C104 to raise your maximum hotend temperature. To apply a configuration file, simply select it in the printer and press the encoder button.

You can get your current configuration using the G-code M503 note only the values relevant to you and copy them in the configuration file.

For have a description of what each G-code means, refer to Marlin's standard g-code documentation, also review the special custom g-codes.

If the file config.gco is renamed to auto0.g in the SD card it will be loaded automatically at the next boot.