It is worth contrasting watermarks with another technique that serves a somewhat similar purpose: signatures. In both physical and digital forms, the basic difference is that a watermark is harder to remove than a signature. A digital file with a specified format can have a digital signature appended to the end of it; in this way, the signer purports: "I (signer) agree to/authorize the content/meaning of this digital file." But it is simple to utilize the digital file and discard the signature; doing so removes the claim made by the signature (in the same way that scissors can be used on a signed sheet of paper). A watermark is much more closely tied in with the file; ideally you would not be able to remove the watermark without altering the content in any obvious way (this doesn't hold true in the paper/scissors example, and some watermarks are designed to photocopy in a way that makes copying evident). Of course, if you have the option of defining what constitutes a valid digital file format, you can explicitly specify that it include a digital signature (from a certain party, if necessary); a file without a signature can be considered automatically invalid by an application (or operating system).