An Extranet is a part of your intranet made available via the Internet. I dislike the term extranet even more than intranet, but suspect we are stuck with it as well. Not all employees are in an organisation's buildings on the corporate internal computer network. An extreme case of this is deployed military personnel. They require access to internal resources via a public communications network, while still keeping the information private.
Defence uses encryption devices to provide secure access to internal systems from remote locations.
It is important to keep in mind that document management is not an organisation's primary business. Enough document management is needed to facilitate business needs, but no more. This is particularly the case with military systems. A combination of simple technical sophistication, plus the good sense and training of staff is required.