What does the `open` keyword mean in Swift and how is it different from `public`?

Hi everyone! I started learning Swift recently and got confused when working with access modifiers. Apple's documentation says that `open` is the highest access level, but I don't fully understand how it differs from `public`.\n\nI'm writing a small library for working with networking. Here's an example of my class:\n\n```swift\npublic class NetworkManager {\n public func fetchData() {\n print("Fetching data...")\n }\n}\n```\n\nEverything works, but I read that for classes that can be subclassed from another module, you need to use `open`. I tried replacing `public` with `open`:\n\n```swift\nopen class NetworkManager {\n open func fetchData() {\n print("Fetching data...")\n }\n}\n```\n\nI didn't notice any difference in behavior. Please explain:\n1. What is the