A smaller area of your choice is selected and cropped .The area is then framed by the correct latitude and longitude .The title and other relevent information such as scale and depths can be  moved around to suit the new chart.

The illustration shown here is a chart of the North Channel and The Firth of Lorn

as you can see at least seven options can be created .

The charts I use are mainly Admiralty and Imray Charts either full size or from their  smaller Folio Series charts