Revisions are managed in the Project Explorer interface via the Revisions tab at the bottom of the interface.
Revisions are a way for you to track changes in an SI 2016 Project. A Revision is a copy of the Project file at "that state in time". The primary reason for creating Revisions is to be able to compare two Revisions of a Project and create a Change Order report.
Every Project starts at Revision 0, and then counts up from there as Revisions are created:
Revisions are intended to be used after you have "sold" the Project and you want to then track changes between the "as sold" and the "as built" Project.