What is mandatory spares?

Mandatory spares" usually refers to spare parts or components that are essential to keep equipment or machinery running without interruption. These are parts that are either:

Legally or contractually required – for example, some industries (like power plants, oil & gas, aviation, or defense) specify a list of mandatory spare parts that must be stocked on-site to comply with regulations, safety standards, or maintenance contracts.

Operationally critical – parts that are most likely to fail during normal use or are vital for the safe and reliable operation of a machine. Without them, the equipment would be unusable, unsafe, or production would stop.