Forwarding methods (as described in Message Forwarding) and dynamic method resolution are, largely, orthogonal.
...
If you implement resolveInstanceMethod: but want particular selectors to actually be forwarded via the forwarding mechanism, you return NO for those selectors.
...
The new code is incorporated into the program and treated identically to classes and categories loaded at the start.
...
For example, the various modules in the System Preferences application are dynamically loaded.
...
Others can write modules that your program loads at runtime—much as Interface Builder loads custom palettes and the OS X System Preferences application loads custom preference modules\
...
They contribute to it in ways that you permit but could not have anticipated or defined yourself. You provide the framework, but others provide the code
...
See the NSBundle class specification in the Foundation framework reference for information on the NSBundle class and its use.
...
