More accurately described as homing switches for this application, these "limit" switches are used when resetting the machine's position. Most any type of limit switch will work, but I like these both because they are cheap and because they don't have the extra metal "trigger" strip like some limit switches. Depending on your set up, this strip can accidentally get bent and ruin your careful setup.
The limits should be wired as normally open when used with the Sun Harvester Shield so make sure you use the leads labeled "NO" and "C".
These limits come in sets of two, enough for one machine.