Amazon Alexa Skills Kit: How do you link with external app account / userId

In an Amazon alexa skill request there is a userId and I'm trying to understand what this is and if there is some reference for it because I want to link and Amazon Echo user account with an account in my own app and to do this I would have to have some kind of static userId to work with.

Example request:

{
 "version": "1.0",
 "session": {
   "new": false,
   "application": {
   "applicationId": "amzn1.echo-sdk-ams.app.[unique-value-here]"
  },
  "sessionId": "session1234",
  "attributes": {},
  "user": {
    "userId": null //IS THERE A DETAILED REFERENCE OF THIS SOMEWHERE?
  }
},
"request": {
"type": "IntentRequest",
"requestId": "request5678",
"intent": {
  "name": "MyColorIsIntent",
  "slots": {
    "Color": {
      "name": "Color",
      "value": "blue"
    }
  }
}
}
}