Definition: msg_value — Allowed: read-only variable inside onmqtt; Not allowed: assignment.
Helper numeric value inside onmqtt. Mapping: true/on -> 1, false/off -> 0, numeric payload -> parsed number (int/float).
If parsing fails (for example malformed text), msg_value keeps the previous valid value.
onmqtt("plc/sp") {
if (msg_is_number) {
setpoint = msg_value;
save(setpoint);
}
}