For example.....an airconditioning MEL has been applied......next to the switch that controls the system a small, normally yellow, sticker, sometimes with INOP or similar already on it, is placed. On the sticker is written 21-XX. The MEL number. 21 is the ATA chapter, 21 is airconditioning, 22 Autopilot, 23 Communications etc up to 80.
This alerts the pilot NOT to use the switch.
You may hear pilots sometimes referring to an aircraft that has "alot of yellow stickers" or 'too many...".