He probably means Mal Ind Lights.
There is only one release callback: SimMalIndLightsOFF
It exists because IIRC a pitbuilder requested it for his rl panel.
That was before I joined the team in 2012.
IMO with the current solutions (DX press / release events, or vendors programming software) it is outdated and not needed anymore. Apart from that the press and hold functions we have do exactly what airtex describes. There is simply no need for the release callbacks.
Btw. and off topic:
I would never(!) setup my full scale cockpit with keystrokes. DX is the way to go.