The unique constraint takes all resources (within the filter if provided) together and checks if each of them has some value that is unique compared to the same value of all the other resources.

- # action: unique # Can be added for consistency, but not necessary
  status: Testing if all canonicals are unique
  filter: StructureDefinition
  unique: url

The action here is optional because it’s implicitly given by the unique property. So the minimal set of the above example would be:

- filter: StructureDefinition
  unique: urlUnique