In addition to notarization, Catalina adds additional requirements. The software and installer must be 64-bit and Code Signing is required for some applications.

Software created before Catalina was not notarized. Following the Catalina release, some developers now send their applications to Apple for notarization.

1. Download the installer.
2. Select Installer in Downloads folder.
3. Hold down Control key while clicking on Installer.
4. Choose Open from popup menu, then click Open in presented dialog.

Note: For backwards compatibility, you may also need to use AllowApps.