Chain link rail ends, or rail end cups, are designed to attach your horizontal rails, top rail and brace rails, to your chain link terminal posts. Rail ends connect to your posts with brace bands and carriage bolts, and you need one rail end per rail to terminal post connection.
Rail ends come in both aluminum and steel. Aluminum is most commonly used on residential chain link projects, while steel is most common for commercial / industrial projects. If you are using a brace and truss setup on your terminal posts, then you should order a combo rail end, which has a hole in it that lets you tuck the tightened truss rod back into the brace rail.