Files that contain the .diff file extension are most commonly used for difference data files. The DIFF files contain the patch script information for the difference data files that are saved with the .diff file suffix. These files are used to show the differences between two different files, such as the difference between a former and later file version.
These changes that are logged in a DIFF file are shown via per-line changes for text files and the output can be applied using a Unix program patch. This is why files with the .diff file extension are sometimes referred to as "patch" files.
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