Idaho Code § 42-904: Division of land into classes by priority.

When any ditch, canal or reservoir delivering or distributing water to several users has one or more rights or priorities by reason of enlargements made from time to time, the right of the land being irrigated by such works shall be divided into classes; rights of the first class belonging to those lands reclaimed between the dates of the first and second priorities or rights of such works; rights of the second class belonging to those lands reclaimed between the dates of the second and third priorities of such works; rights of any other class being determined in like manner; but all the rights belonging to the same class shall be equal and subject alike to the regulations of their respective class.