Studying the geography of wine you may meet statistics concerning the percentage of plain, hills and mountains in a determined area.
The question is: under which conditions a plain is a plain, a hill is a hill and a mountain is a mountain?

In Italy these values are determined by ISTAT (Italian National Institute of Statistics) which also gives us official definitions reported by various governmental websites (like the one of Valle d’Aosta region).

According to ISTAT, a plain is a stretch of land devoid of any notable rise or elevation, including inland portions farther from the sea that reach up to 300 meters, as long as their inclination is negligible.

A hill is a portion of land characterized by the presence of notable and distinguishable rises or elevations reaching an altitude of up to 600 meters in central and southern Italy (including the islands) and 700 meters in northern Italy. Notice that according to these definitions the difference between a plain and a hill is not only a matter of altitude, but also of topography: there can exist hills 200 meters high, provided that the mass of land is relevant and distinguishable. I could not find the exact criteria that may be applied in borderline cases and the exact difference between a negligible inclination and a proper slope (in terms of degrees). I imagine that it often goes case by case.
However notice also that if a single hill or a small hilly area of limited extension is included in a wide flat plain, it would be counted as plain as well. On the contrary small flat areas included in wider hilly systems, like valleys, or limited coastlines overlooked by hills will be by convention included in the hilly area. This is why you have virtually no plains in Marche region, while – you know – we didn’t built the Ancona airport over a slope, it is on flat land. But by convention this flat land, being limited, is counted as hill.

How small must a coastline or a valley be to be included in the hills? How limited in extension must a group of hills be to be still considered plain? Only ISTAT knows.

Finally, a mountain is a portion of land characterized by the presence of notable and distinguishable rises or elevations reaching an altitude of at least 600 meters in central and southern Italy (including the islands) and 700 meters in northern Italy.
Areas of limited extension bearing different characteristics (for example plateaus, valleys or narrow coastlines) included and surrounded by mountainous systems are counted as mountain land as well. This is for example why Trentino Alto Adige is 100% mountains despite the Piana Rotaliana and other valleys.

I hope this will help you in the study of Italian wine.