SBS-1
In answer to the orginal question - unverified as described by Chilli and another, is more a concept used by controllers in the UK to understand whether parts of the secondary surveillance radar data shown on the display needs to be checked by that controller or whether he/she can assume that they are correct. It is not a concept that is implemented in any radar display system that I have seen although I guess it could be a default value used until a positive action is taken to check the data. Having said that, I'm an approach person myself and en-route radar systems are often more sophisticated when it comes to processing data so if your source is an en-route RDP system maybe this is what it's about.
If you want anything more definitive you'll have to be more specific about where you are looking at the radar and what the equipment is.