Adobe Photoshop General Usage Notes
Adobe Photoshop requires the use of a special file, called a “scratch disk”. This file is used as a caching location, much like web browsers use, to temporarily copy an item so that some operation can be done on it. For instance if you edit a photo to shade it, the photo is copied to the scratch disk, the shading operation is done to it there, and the results displayed. Once you commit to an operation that scratch file becomes the original file.
Photoshop can also be set up to perform batch operations. This allows a directory that is indicated, with images in it, to be processed exactly the same way and then the results deposited to another directory. This automates routine tasks, such as image sizing.