Yes, in this case it is the general catalog of sets. I'm writing an iOS app that allows the user to keep track of their collection. I know this duplicates the functionality of other sites and apps, but I am writing this as an exercise as it ends up covering a lot of iOS development I want to practice.
I was planning to seed the app with a database containing information from Rebrickable so the entire data does not need to be downloaded on first use. To limit the number of requests to Rebrickable and to restrict bandwidth to the minimum, I would like to be able to do an occasional search that will return the items in the local database are out of date and the new sets in the Rebrickable database (based on when the local database was last updated).
The alternative is to download the sets csv file and work out what is new (and deleted) based on that without knowing if any information has been modified on existing sets.
I hope that adds clarity to my request.