When you register device on azure push notification service - it creates new entity on its notification hub.
This entity has 3 main fields: registrationId, deviceTokens and tags.
Test send works with tokens. Tokens are just like tags in Instagram or Stackoverflow. You can pass there something like user name and send on all user devices by one send.
Also you can pass device token in tag, or regId. So you can test send test notification on selected device.
P.S. Best way to work with azure services: Visual Studio. You can connect your azure hub, watch the list of your registered devices (regId, deviceToken, tokens and expiration dates(you can not do it in azure console)) (also you can use test send by VS)