I'm not sure I follow your question. If you're refering to the "dual filament" versus the "single filament" bulbs, such as those listed on
Lockhart Phillips site , the difference is that the dual has an extra filament to allow the bulb to be on as a marker light (such as your tail lights), whereas the single only has one filament to flash for signals. Of course, the dual filament bulb requires an extra wire to serve the marker light.