Digital signatures become part of the design file. In fact, a digital signature is really an element in the model to which it applies. File signatures are elements in the internal "settings" model. Making the signature a part of the file makes it impossible for a signature to somehow be detached from the design accidentally, as can happen when a signature is applied externally to a file.
Promis.e does not require a different file format, special packaging, or special viewing software to associate signatures with design data.
Promis.e automatically validates digital signatures. In other words, you cannot look at a signature without seeing that it is valid or invalid.