When you upload a file to the Walrus platform, it returns 'newlyCreated' if the file is being stored for the first time. This means your file did not exist on the Walrus storage system before, so a new blob is created and certified. On the other hand, the response 'alreadyCertified' indicates that the file has been uploaded previously and a certified blob with the same ID already exists. Thus, it does not need to be stored again, which optimizes storage space.

If you want more details on how these optimizations work, you can check the additional info provided in the Walrus documentation here.