title: What is GCM Registration ID?
body: <p>Hello I want to add gcm to my application. My application is a news application. 
  User can view news after registration. Application asks user interests (economy, sports, cars etc.) and saves them.
  I want to send news to user as notification from php and mysql. 
  I must decide to send which news to which users. So i need registration ids. </p>
  
  <p>Is it android device number? 
  Or a combination of Android device google account and android device number?<br>
  Or a combination of Android device google account and android device number and My application user id?</p>
  
  <p>Example</p>
  
  <blockquote>
    <p>Android device id = a => GCM RegID=a <br> Android device id = a ,
    google account id=b => GCM RegID=ab<br> Android device id = a , google
    account id=b, application x user id=c => GCM RegID=abc</p>
  </blockquote>
  
  <p>If x user logs out from application and y user login in same device will GCM RegId change? If it does not change and i push economy news to y user (who does not interest about economy) device will show wrong notification to wrong user. 
  So what is GCM registration id? </p>