These are specially designed fence kits that include protection against climbing animals (in, out or both ways in and out). The posts in these kits include extensions on the top of each post; when paired with included fencing, these extensions make an overhead canopy that animals can't climb over. These kits can include overlap and a rodent barrier. They are most often used by customers that want to keep their cats in and other animals out.
Size example: 7 1/2 x 300 refers to a fence kit 7 1/2 foot above ground finished height that is 300 linear feet long . Linear footage is a measurement of footage as if you measured with a tape measure. If you have a large area to measure, measuring wheels are available in most hardware stores. Have a surveyed map of your property? Most have linear footage that you can scale from with a ruler.