Old youtube tutorial: Quick Tip: Resume Render Feature
To save the content of the VFB in order to continue rendering it later:
1. Start rendering
2. Stop rendering by pressing "Stop" button in the VFB or in the Rendering window.
3. Go to Render Setup > Scene > General Settings and press "Save CXR..."
4. Save the Corona EXR file in selected location
5. To continue rendering press "Resume from file..." and load the previously saved CXR file.
Note: if you have stopped your rendering and you want to continue it without saving the CXR file, you can use the "Resume last render" button.
Note #2: this feature will only work with Corona EXR files. It will not work with a "regular" .exr file saved from the VFB.
You can also continue rendering autosaved images - see: What is "Autosave" feature for rendering?