FBZ6 is the compressed project file format for FinalBuilder, it’s basically just a FBP6 file that’s stored in a zip archive. If you are storing the project file in version control then a good idea is to change to the uncompressed format (fbp6) as it can be more easily diffed. You can change the project to FBP6 by going Save As and selected the correct format. If you need to diff the FBZ6 files then you will need to rename both to have a zip extension, extract them and compare the fbp6 files.