Incandescent dual filament bulbs such as the 3157 have two separate filaments that are on different circuits although they share a common ground. The dim filament is used for the parking/running lights controlled by the headlight switch. The bright filament is used for the turn signals (and in some cars the DRLs). But both are the same color because the color is determined by the glass globe of the bulb rather than by the individual filaments.
Switchback LEDs take advantage of the unique ability to combine different color light sources in a single housing. They have white LEDs connected to the parking/running light circuit and amber LEDs connected to the turn signal circuit. Since they distinguish the two circuits by color, it is not necessary to have one brighter than the other so most switchback LEDs have the same output level for both white and amber.
The two types of switchback bulbs are known as Type 1/Type 2 or AW/AO based on how they function when the turn signals are in use. If the parking/running lights (white LEDs) are off, both types operate identically. When the parking/running lights are on, Type 1 or AW switchbacks will alternate amber and white when the turn signals are flashing. Type 2 or AO will flash amber only until the turn signals are finished and then switch back to solid white. (AW = amber white and AO = amber off). I have not seen the terms A Type and B Type but I would assume they mean the same thing. Type 2 (AO) function the way factory LED setups are designed while Type 1 (AW) seems to have originated with import car "tuners" (also known as "ricers").
There are a few unique switchback bulbs that have special circuitry designed for use in factory DRL applications. They are usually Type 2 (AO) but have additional circuitry to switch to white when the turn signal circuit shows constant (not flashing) power. This gives white DRLs while still retaining amber turn signals. They only work in vehicles that use the front turn signals for DRLs as well.
Edited by TheWizard, 03 August 2014 - 08:33 PM.