When code does not fit in an SPE's local store, overlays can be useful.
An overlay is SPU code that is dynamically loaded and executed by a running SPU program. It cannot be independently loaded or run on an SPE. SPE overlays allow the programmer to manage SPU code in a modular fashion.
The specific SPU code that is needed at runtime is dynamically loaded.
Additional information on developing code with overlays is provided in the Software Development Kit, Programmer's Guide.