AFAIK, you don’t need anything else apart from that definition. I assume it was defined that way in case that string changed, since it is used in many components.
PLASMIC_AUTH_DATA_KEY is a local variable used in the examples, it’s not actually something that relates to a configuration in Plasmic studio. In the example, it’s only being used as a key to the cache, so that the user object doesn’t need to be consistently fetched from Plasmic servers.