The Eclipse framework uses the notion of projects that are locally managed files containing local history information and extraneous file attribute information beyond what the underlying filesystem usually provides. It is no longer possible to just open an Acme file in AcmeStudio; that file must be part of a project. A project can be created new, checked out from a CVS repository, or imported from various sources.
A feature of project is that AcmeStudio keeps track of local versioning information about files, regardless of whether those files originally came from CVS. You can compare files with any previously saved version of the file, and also replace the file with any previous version. These can be accessed by right-clicking over the file in the Navigator view and selecting Compare with… or Replace with… from the menu.