By default the recommendation engine have no information about the user visiting the website. The only information provided in the event tracking and recommendation request is the external user ID (usually a Session ID for anonymous and some code like a customer ID for the registered users.) This external ID is additionally anonymized in the recommendation engine.
Sometimes it is needed to store an additional attributes for every user, for example the age or home city. As the external user ID this information is stored one-way. It is possible to retrieve the stored information back based on the external user ID, but it is not possible to get the external user ID, if an attribute set is provided (unless the external ID is not explicitly stored as an attribute of course).
Usage of the user attribute in the recommendation engine is under construction. Right now the only possible way is to activate attribute boost filter (see links below).