Pretty self-explanatory, in my mind. There are plenty of great places to plant letterboxes, to be sure! And if you get the urge to plant a letterbox in a park that already has one or two letterboxes resident, it is good form to speak to the planters prior to placing yours, just out of courtesy. The only time this wouldn't be necessary is if the park is very large and able to accommodate many 'boxes without feeling crowded. (This is a good general rule to follow; however, there are always exceptions.)
This bit of good advice is another hitchhiker that's out traveling about the country, measuring 2" x 1.75".