Some time ago I found a post that showed how to generate temporary images, a gray background image and a background mark containing the dimensions of the image. This image is generated automatically, with dimensions determined via pass parameter get, that is, it was not saved to disk, it only served to have a preview of how the layout would look.
However, I lost this post and can not find anything to talk about the feature. Any guidance on how I can use this feature? Documentation, etc.?