With "unlimited sheds", the string power decreased linearily while the buttom cell row is being shaded. The beam component of radiation is reduced by the ratio of the shaded cell area to the full cell area. This is the correct electrical behaviour if unshaded and shaded strings are connected in parallel (MPP voltage is determined by the unshaded string, no bypass diodes are involved) With 3D, the irradiance of the buttom string will be reduced immediately by the beam component as soon as the buttom cell row is affected by a shadow. In PVsyst 6.0, "According to module layout" combines the best of 3D and "unlimited sheds".