C-Bert
Secondary Surveillance Radar is the generic term used to describe most civilian systems which allow for Mode 3 and Mode 'C' to be displayed to our ATC brethren. I believe that ATC don't generally use Primary Radar data anymore but display SSR plots only?
As Wader quite rightly points out the Ident Friend or Foe gives more information to military users in an operational environment.
Therefore you will normally find SSR/IFF quoted together when talking about military Search Radar sytems as we Mil types have to see our civvie mates in a routine training environment.
The Mil AIP and JSP552 are 'right rivetting reads'and will give you all the information you require about ICAO classification of airspace plus the rules and regulations we have to abide by. However, if we lose SSR/IFF during mid-vul and depending upon what type of airspace we are operating in, then we can simply limit, downgrade or change the radar service we have applied or reduce the altitude blocks allocated to the assets as necessary.....it really is that easy!
SA