G18 means “all commands are now to be interpreted in the XZ plane”.
This command essentially changes the meaning ( or plane ) of the I, J and K parameters of the G2 and G3 G-codes.
Smoothie starts in the G17 mode ( the default plane is the XY plane ). You can change this with the G18 or G19 commands, and you can come back to the “normal” plane with the G17 command.
This command is modal, meaning that once it is executed, all Gcodes after it are now in this now “mode”, until changed by another mode-changing Gcode of it's family.
The command is used as such :
Which means : G2 and G3 commands are now in the XZ plane.
These resources are used as references for Gcode :