https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/AccessControl.html#//apple_ref/doc/uid/TP40014097-CH41-ID3
Open access applies only to classes and class members, and it differs from public access as follows: - Classes with public access, or any more restrictive access level, can be subclassed only within the module where they’re defined. - Class members with public access, or any more restrictive access level, can be overridden by subclasses only within the module where they’re defined. - Open classes can be subclassed within the module where they’re defined, and within any module that imports the module where they’re defined. - Open class members can be overridden by subclasses within the module where they’re defined, and within any module that imports the module where they’re defined. (edited)