A VC exists only in the Lightroom catalog; it’s simply another instance of the original photo within the database. When you make a VC, there’s no file actually saved on the hard disk unless you Export the VC as a “real” image file. The key benefit of VCs is that you can make multiple variations of an image without taking up any additional disk space.
In general, once you have “finished” a master photo and applied all the adjustments to make it look the way you want, any further modifications for specific purposes (such as printing) should be done using VCs.
For example: many printer/paper combinations produce printed output that is darker and less saturated that what you see on screen. You can make a VC with adjustments to compensate.
To make a VC, right-click or control-click on the photo, and from the popup menu, select Create Virtual Copy. Or press cmd’/ctrl’ (apostrophe). Or choose the command from the Photo menu. The VC will be created in the same Folder and/or Collection as the original with the same adjustment settings as the original. And you can make a copy of a copy with no loss of quality.
In the Library Grid, a Virtual Copy is indicated by a “page turn” icon on the thumbnail.
Once you’ve created your VC, you can adjust it for the print job. Using the previous example, you might increase Brightness, Fill Light and Saturation to improve the printed output.
TIP: Use Quick Develop in the Library module to apply adjustments for print. Quick Develop adjustments are relative, meaning they are applied on top of the values of any existing adjustments.
You can also use VCs for alternate crops/aspect ratios, making triptychs and multi-image layouts, etc. … all without affecting your original master photo.