Files with the .MD5 extension contain a control sum, which is a string of numbers created using the MD5 algorithm based on the contents of another file. In the MD5 standard, it is a string of 32 characters with 128-bit coding. The control sums are used for verifying the integralness of a disc or its image, or of downloaded files. They are often created when generating a disc image.

Comparing the generated MD5 value with the original value lets the user verify, whether the image was generated correctly.