If a managed package is present, which Apex classes can be secured?

When a managed package is involved, the security of Apex classes is determined by their access modifier. Classes that are declared as global can be accessed by any other code in any namespace, which makes them suitable for inclusion in a managed package. Additionally, classes containing webservice methods are also implicitly allowed because they need to be accessible from outside the package for integration purposes.

This means that to ensure proper security and access, the managed package allows developers to secure only those classes that are necessary for external interaction, and thus global classes or those with webservice methods are the focus. This is fundamental in managing how external applications can interact with the package and ensuring that the internal logic remains secure. Therefore, the specific declaration of global or webservice methods indicates an intention for wider accessibility, which aligns with the requirements of the managed package in a secure manner.