What does Combine Rotators do ?

[PenguinTD]

they are internally quaternion, here is a snipped of code from engine source

FRotator UKismetMathLibrary::ComposeRotators(FRotator A, FRotator B)
{
    FQuat AQuat = FQuat(A);
    FQuat BQuat = FQuat(B);

    return FRotator(BQuat*AQuat);
}