Cargo loaded to the open railcar is oversize if its overall dimensions exceed limits of the loading gauge (or its any oversize part exceeds geometric displacement of the designed car on curve track) providing the car is on the straight and flat track (car and track long axes coincided in vertical plane). To verify the freight meets loading gauge, it is necessary to consider that the freight loaded on the car with design height of the floor 1400 mm from the railhead.

Three basic areas of oversize are adopted depending on the height (from the railhead level) at which freight exceeds the loading gauge:

- lower oversize area - at the height 480 - 1400 mm and at the distance from track axis 1626 - 1760 mm, and at the height 1230 - 1400 mm and at the distance from track axis 1761 - 2240 mm;
- side oversize area - at the height 1400 - 4000 mm;
- top oversize area - at the height 4000 - 5300 mm.

In addition, combined side and top oversize area, at the height 4000 - 4603 mm and at the distance 1625 mm from track axis to the limit of top oversize area, is used for more accurate determination of requirements for transportation of top oversize cargo on a double track line.

Depending on a degree of exceeding the loading gauge, the following degrees of oversize are applied to the basic areas:
- lower oversize area - six degrees;
- side oversize area - six degrees;
- top oversize area - three degrees.

The freight is extra oversize:
- if exceeded limits of top oversize of the 3rd degree, side - 4th (height 3700 - 4000 mm), 5th (height 3400 - 3700 mm) and 6th degrees, lower - 2nd (height 380 - 1230 mm) and 6th degrees, in horizontal plane;
- if exceeded limits of lower oversize area, 3rd - 6th degrees and extends over the loading gauge (height above 5300 mm), in vertical plane.

According to the areas of the oversize, the freight can be lower, side or top extra oversize. The freight with height over 5300 mm is vertical extra oversize.

Index of five symbols is used for description of the oversize:
1st symbol is always Russian letter "H" (N),
2d - lower oversize area (degrees 1-6),
3d - side oversize area (degrees 1-6),
4th - top oversize area (degrees 1-3),
5th - vertical oversize.
Symbol "8" for any area means extra oversize.

For example:
H-3-5-2-8 means combined oversize (lower of 3-d degree, side of 5-th degree, top of 2-d degree, and vertical oversize).