This reference surface area was implemented by the reform on structure control in the 1999 Law on Agricultural Orientation (Article L 312-5 of the Rural Code):
The Unité de Référence or U.R. (Reference Unit) corresponds to the surface area “that ensures viability of the farm, in view of the type of crop and non-land production workshops, as well as other agricultural activities”.

This is a standard that substitutes for the S.M.I in structure control rules.

> The U.R. is the basis used to define thresholds used for structure control.

> For preferential allocation of rights to produce, the farm must not exceed a certain threshold, based on the U.R..

The U.R. is calculated according to the average surface area of installations over the past five years in the county.

The U.R. is set by the administrative authorities after advice from the County Agricultural Orientation Commission (C.D.O.A), in the County Structural Plan (S.D.D.S), and for each natural region in the county.

Examples: U.R. = 30 ha in Moselle, U.R. = 50 ha in plain zone in Vienne.

The U.R. was created solely for application within the context of structure control.
The U.R. is not based on the sole consideration of surface area, it is directed more towards an economic approach. This also has an agricultural industry value (the U.R. varies according to the natural regions in the county).
The SMI is always used for social criteria: M.S.A., pension, etc
The U.R. is equal to around 2 SMI in most cases.
 
If the farm is located across several counties with different U.R. and S.M.I., the share of each is calculated (in proportion to the surface areas involved).