If an application isn't able to preserve unknown properties, then it has to display a warning message. Illegally formatted game-info properties should be corrected if possible, otherwise preserved. Other illegally formatted properties should be corrected if possible, otherwise deleted. An application has to display a warning message, if it deletes illegally formatted properties. Private Properties Applications may define their own private properties.
Some restrictions apply.
You have to use a new identifier instead. The identifier should consist of up to two uppercase letters. SGF doesn't require to limit the identifier to two letters, but some applications could break otherwise. Otherwise the file would become unparseable. Should the value type be a combination of two simpler types then it's suggested that your application uses the Compose type.