Location indicates the specific point in a publication at which the item being referenced is located. Location is required. This can be:
Extent refers to a number that indicates the total physical extent or size of the work. Extent is optional. This can be:
Place the location after the volume and issue numbers. Use a colon before the location, with no space between the colon and the location and a period after the location.
From CSE Manual 9th ed., Ch. 29.3.3.8 Location within a Work and Extent of a Work (Pagination)