Ordinarily, bows tillered for split have a slight positive tiller, maybe 3/16. Bows tillered for 3 under have less of a positive tiller, or sometimes have zero tiller. The reason is that the pulling force is exerted lower on the string for 3 under, putting more force on the lower limb. The objective is for both limbs to recover at the same time.
The thing I don't understand is that for many years I have requested that all the bows I buy to be tillered for 3 under, but the actual nock point required to shoot 3 under has varied from 3/8" to 7/8", based on shooting a bare shaft and tuning for a very slight nock high flight. My impression is that it's better to be shooting at a 3/8" nock height than a 7/8" nock height, assuming a tuned bare shaft in both cases, but I'm not sure if that's true or not either.