Change type: "added", "modified", or "deleted". User-friendly: "file X was deleted", "file Y was modified", "file Z was added".
Type changes (file→folder, folder→file) are just delete + add, not a separate action. Note: Renames use change_type="deleted" with moved_to filled in (file was deleted from original path, moved to new path). Note: Renames also use change_type="added" with moved_from filled in (file was added at new path, moved from old path).