A package bundle is a collection of two or more packages that can be pre-configured and used for provisioning a server.
Default Package Bundles are collections of packages specifically configured and put together by Bunnyshell to either serve a specific application (like Wordpress), or a generic function (like a web server). Check out the reference page:
Yes. Check out: Creating a Package Bundle
Yes. Go to sidebar -> OPS -> Package Bundle, find the one of interest, hit the Action button on the right and chose Edit.
No, Package Bundles are OS-dependent.
No, Package Bundles can only be used when creating a new server.