How I understand the way it works in real life:
VEI: "Veille" in french / "Standby" - System is off
[]: "Normal" / - System is active and you can turn jammer on/off with the HOTAS button.
PCM: "Priorité Contre-mesure" / "ECM priority" - Jammer active but alterate/jamm your own radar. You can still turn it on/off via HOTAS.
Like that:
Notice some differences on status lights between this one and the actual one we have.