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The following picture shows the engine throttle of an Aermacchi MB-326, which is powered by the Rolls-Royce Viper turbojet:

(source - all photos © Drive&Fly Model Club)

The engine throttle on board the MB-326

I wonder what exactly the RELIGHT push-button is for. It's clearly not the 'reheat' control (another word for afterburner, which is not fitted on the Viper, apart from some amateur variants...)

I would guess it controls the engine ignition for use in case of flame-out, but then I noticed this other photo, showing what appears to be the starter/ignition controls (located in front of the throttle):

MB-326 ignition control

so my question is: what exactly does the relight button do, as opposed to the starter/ignition controls?

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enter image description here

The Aermacchi MB-339 was developed from the -326 and it is also powered by one Rolls-Royce Viper Mk. 632 turbojet. From its aircraft manual, the relight button is for the in-flight re-ignition:

Premuto - Attiva il sistema di riaccensione in volo.
Pressed - Activates the re-ignition system in flight.

The start button in your picture is for activating the starter, and the guarded ignition next to it is likely a ground/start-only ignition mode.

Typically, an in-flight ignition mode is of a lower energy rating compared to the start mode, this helps prolong the life of the igniters. It's used when flying in rain or icing conditions, and for maneuvers that could momentarily choke the engine (e.g., an extreme spin or side-slip with side-mounted intakes).

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Early turbojets such as the Viper had a tendency to flame out. The relight button was used to restart the engine in the air.

Source

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