When using Edlink's API there will always be a Submission for every Enrollment in a class. This is true even if the student has not submitted any work for an Assignment.
In order to support this concept in LMS's and SIS's that don't have a default submission for every student in a class, Edlink will create a phantom submission for every enrollment in the class missing a submission. This phantom submission will be in the created state and will have no attempts.
These submissions can be treated like any other submission. Edlink will ensure it gets created in the LMS/SIS when submitted by the student. The UUID will not change once you've seen it, you can safely store a phantom submission's UUID just like any other submission. The only difference is that the submission will not be visible in the LMS/SIS.
A few providers that use phantom submissions are: Canvas, Aeries, Clever, and OneRoster