In short
The outline of a piece of land is drawn by the cadastral map of the Agenzia delle Entrate (Italian Revenue Agency), which is public. If you are on site, your phone's position is enough to know which parcel is under your feet. If you have sheet (foglio) and parcel (particella) from a cadastral record or a deed, enter them and the boundary appears, with the area.
You are on site
Use your phone's position
From your phone's position, the parcel under your feet.
The position stays on your phone: only the point reaches us, and we do not keep it.
Next step: PDF report €1.90 · cadastral record €4.90
What you see, and how precisely
You see the polygon that the Agenzia publishes for that parcel, with its area and the neighbours around it. It is the drawing surveyors work with: it shows where your piece ends and how many parcels make up a holding. To drive in a boundary post you need a survey.
On site, with your phone
The phone says where you are, the map says which parcel that point belongs to. It is useful when you are looking at land you do not know, or looking for a boundary that has vanished under the brambles.
When the numbers do not match
Subdivisions and mergers change the numbers: it is explained in What is a cadastral parcel.
Land made up of several parcels
A farm holding is almost never a single parcel: often there are ten or twenty, sometimes on different sheets. On the map you select them all and they stay together in the link, so the outline you see is that of the whole holding.
If the municipality uses the tavolare system
In the provinces of Trento and Bolzano the tavolare land register system applies, and the maps are in the provincial archives: those parcels are not in the Agenzia's archive, and when that happens we say so.
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Find your municipality
Outlines and areas: cadastral cartography of the Agenzia delle Entrate, CC-BY 4.0. The area is the cadastral one and does not replace a survey. The provinces of Trento and Bolzano use the tavolare system and are not part of this archive.