The definition of “document” is the same one that appears in the Criminal Code. I don't know the exact section, but it's the definition used in the Criminal Code. “Document” refers to any evidence or piece of evidence on any medium, computer-based or otherwise, on which data is registered or marked, and that document is sealed. If the document is a hard copy, it is placed in a packet and sealed. The same goes for a computer-based device. What matters is that the item is sealed to prevent police or the individuals executing the warrant from accessing it before the journalist or media outlet has been notified. The journalist or media outlet could then advise the individuals seeking the information that the computer or hard drive contains a multitude of other documents that have nothing to do with the case in question but that could be considered information likely to identify journalistic sources.
For a model, I relied heavily on the procedure used in the case of law firms. When a search warrant is executed on a lawyer's office, the information is protected by solicitor-client privilege and steps are taken to ensure that privilege is not violated.