What is styleData in Dashboard example?

[HolyHigh]

hi, everyone. I am new to QML, and learn QML through its example.
When I look through the Dashboard example, I find there is a variate named StyleData which I can not find its declaration.

//dashboard.qml line 140
style: IconGaugeStyle {
 id: tempGaugeStyle

 icon: "qrc:/images/temperature-icon.png"
 maxWarningColor: Qt.rgba(0.5, 0, 0, 1)
 tickmarkLabel: Text {
    color: "white"
    visible: styleData.value === 0 || styleData.value === 1
    font.pixelSize: tempGaugeStyle.toPixels(0.225)
    text: styleData.value === 0 ? "C" : (styleData.value === 1 ? "H" : "")
  }
......
}

When i change the code into the follwo, it doesn`t work well .

text: test1(styleData.value)
function test1(datas){
                    if(datas===0) return "E"
                    else if(datas===0.25) return "M"
                    else if(datas===1) return "F"
                }

the gauge will not show these letters.
So, I want to ask what is the styleData and What should I do to use it.

[Markkyboy]

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