The Unrestricted SVMO requires you to complete an oral examination on general engineering knowledge as per the syllabus. In certain situations an Unrestricted SVMO can be the engineer on vessels with up to 3000 Kw of propulsion, so you need to be familiar with large diesels, and the pumps and systems that go along with them.
The Restricted SVMO certificate uses the same syllabus, but the practical test is specific to the vessel(s) that you will hold the certificate for, and you will need to re-test for each individual vessel.
The seatime requirements and what onboard roles you can fulfill also differ between the two licenses.
General certificate information (Page 306, Chapter 33)
Certificate Requirements (Refer to Section 151)