A new revision can be created at any time during the opportunity process. When you create an opportunity, a revision is created automatically, but all new revisions must be created manually. Revisions are typically created to keep a history of changes made to the opportunity throughout its lifetime. You can also create a new revision if you want different versions of the opportunity to e.g. offer alternative quotes to your potential customers or clients. When creating the new version, you can decide whether the old version should be obsolete. You can also manually obsolete a revision at a later stage.
An opportunity can have multiple active revisions but one of them is set as the main.