These clip onto 1" metal pole shelving units. There are 2 slots to hold 5-7oz2-3" cups and one that holds 9-18oz4" cups. The insert allows you to use all 3 slots with the smaller cups. Multipurpose clip on tray can be used to catch drips or runoff or to throw your tools on while you work. Of course you could just fill it with plants also 😉 SET Include all 3 pieces *clip on tray *clip on 3 cup holder *smaller cup insert