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Explanation: The fact that it mentions "larghezza" gives me the idea that this item can also have a length. Therefore, I think "dimensions" is not specific enough.
"Utile" is used to indicate the actual area that can be used, although the machine (sounds like a belt in this case) could be larger in terms of the amount of space it occupies.
Here's an example of what a mean (for another field, but the concept's the same):
"Remember, a table with an additional head / face rest adds useful length to a table but a massage table with a built-in face hole considerably decreases the effective working length. For example, the working length of a 6ft (1.83m) table with a face hole 6" (15cm) from the end, is effectively a 5'6" (1.42m) table suitable only for a 5'4" (1.37m) client."
The second site (agricultural machinery) has a table showing the differences between the working width and the transport width.