More about referencing packages
This article describes a deprecated feature as of Panda3D 1.10.0.
A p3d file may include a reference to one or more packages, which means that those package(s) will be downloaded and installed before the application even begins. This is specified with the -r parameter to packp3d, or with the requires() call in a pdef file.
It is also possible to download and install one or more packages at runtime, after the application has already started. To do this, instead of referencing the package with the -r parameter, you can call base.appRunner.installPackage() at runtime, or you can use the PackageInstaller or DWBPackageInstaller class to download and install it asynchronously.