All versions of the RCX have a unique number printed on it , necessary for technical support .
All versions of the RCX used as the ID number of the RCX for your Lego Mindstorms account on the now - defunct Lego Mindstorms RCX website .
All versions of the RCX have a unique number printed on it , necessary for technical support and used as the ID number of the RCX for your Lego Mindstorms account on the now - defunct Lego Mindstorms RCX website .