It
adds new tags for storing the replay gain info. These are like any other normal tags in the files.
By default MC always writes tag changes to the files and creates new file tags when needed (most people expect it to do that), but you can disable tag writing in the field specific options or completely in the general options. MC can work as well by keeping all tag data in its database.
The replay gain system is a two-stage process. You need to analyze the files before you can use the playback correction system. The analysis process decodes the files and measures the track gain and track peak values. In addition MC's analyzer measures the BPM and Intensity values which are not needed for the replay gain system, but might provide useful information. In MC you have two options for analyzing existing files. You can set the analyzer to run automatically in the background as a part of the import process or select the files and run the analyzer manually. In addition, you can set MC to automatically analyze the ripped files in the CD ripping options.
Here's more info about the replay gain system in general:
http://replaygain.hydrogenaudio.org/The system really does not need any support from the file format. Any normal audio file can be analyzed and the measured replay gain values can be used for automatically setting the playback volume level.