Crop Area vs. Crop Marks, What's the Difference?
There can only be one Crop Area in an Illustrator document. It is used to determine the printable boundaries of your artwork when you want the Trim of the art to be something other than your full page. To set up a Crop Area, draw a rectangle to the desired size and position, and go to Object > Crop Area > Make. This will set the Crop Area and automatically places (visible but non-printing) Crops into position.
On the other hand, multiple sets of Crop Marks can be set up within a single document. This is useful in setting up duplicates or altered versions of a design for printing. To set up Crops, draw a rectangle to the desired size and position, then go to Filter > Create > Crop Marks, and Crops will be drawn in position. The rectangle is still there but it will lose any Stroke or Fill. The Crops are not locked, so it’s easy to marquee across any unnecessary interior Crops to select and delete.