Since we want to retain the documents for 2 years and then delete them we want to have a retention label that will get applied to these documents that contains the reference.
To apply the retention label correctly we can use an auto-apply label policy and when configuring it we can add the reference Project Alpha to the auto-labeling policy.
https://docs.microsoft.com/en-us/microsoft-365/compliance/apply-retention-labels- automatically?view=o365-worldwide#how-to-auto-apply-a-retention-label