I have a Canon 550D and in the SD Card I found a file \DCIM\CANONMSC\M0205.CTG
What is this file used for?
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.CTG files contain image catalog information that is used by the camera when managing and displaying photos. You'll probably find a .ctg file for each set of images created.
The .CTG files should get deleted automatically when the associated images are erased, although it seems that current best practices are to format the memory card often so I wouldn't worry about the .CTG files.