If you are offering an unlimited testing facility to your users, we need to be able to identify the number of unique users of your application. This is done by setting a user GUID for each test you create. If you are billing your users exclusively on a per-test basis you can ignore this feature.
You must be certain that the GUID used is unique to that user. The GUID should be stored
in a unique field within your database and should not change for the life of that user's
account.
This information is never shared with anyone, not even your user. It will be used in your billing reports and returned to you along with your test results.
Do not use a hash of your user's details unless you generate this only once (such as
generating it when you create a new user) and you can be certain that this hash will be
unique. This is because a hash of their email address, for example, could change over
time and cause them to be identified twice in one month.
You can find specific implementation details for user GUIDs in the REST and SOAP documentation: