There is only one file type that uses the .MSCZ extension:
MuseScore Compressed Score File
An MSCZ file contains a music score created by MuseScore, a free program used for music composition and notation. It stores a composition, including instruments, time and key signatures, and music notes. MSCZ files also contain score metadata such as composer, title, and copyright information.
You can open MSCZ files using MuseScore in Windows, macOS, and Linux. You can also log into MuseScore online and upload MSCZ files to open and edit the files and share them with other MuseScore users.
MSCZ files are the standard save format for MuseScore. They are stored in a compressed Zip format. Uncompressed music scores are saved as .MSCX files (or .MSC files in MuseScore version 0.9.4 and earlier), which can be edited manually with a text editor.
Updated: January 27, 2019