2 function decoders support F0 Forward and F0 Reverse. This allows for reversing headlights or for two separate functions.
How functions are numbered:
F0 (FWD) White
F0 (REV) Yellow
The Blue wire is Lamp Common to allow for continuous lamp operation.
F4 White with Yellow Stripe
F5 White with Green Stripe
F6 White with Blue Stripe
and so one are each counted as one function. This is an industry wide convention.