Revisions - used to track changes while still in the bidding process
Change Orders - used to track changes a Project once it has been approved
Revisions are a way for you to track changes that are requested to your original proposal to a client while still in the bidding process. For example you may propose one solution to a client and they review it and want to upgrade some of the equipment or they now require additional solutions. You can create a Revision which is a copy of the Project at "that state in time". You then make the requested changes to the Project and re-present to your client.
Revisions can also be compared against each other to view the changes that took place between them.
Revisions are managed in the Project Explorer interface via the Revisions tab at the bottom of the interface.
Every Project starts at Revision 0, and then counts up from there as Revisions are created: