Components can be divided into three categories, based on what you must do to the server before servicing them.
Hot service components can be serviced while the server is powered on and running in full-power mode.
Warm service components can be serviced while the server is in standby power mode. These include CMODs, DIMMs, and processors and heatsinks.
Cold service components must be serviced when the server is completely powered off and disconnected from the power source.