By default, each new Ultiboard project will have trace biases that alternate between horizontal and vertical with each successive layer. This setting greatly increases routing speed and completion rates and should not normally be changed. The exception is if you have placed several powerplanes. Because the router does not route traces on powerplane layers, these layer biases may need to be adjusted to preserve this alternating pattern so that it skips over those powerplane layers.
This information can be found at Ultiboard User Guide