question about keypoint

[little-misfit]

thanks for your work！In the keypoint feature, the first two elements of each point are coordinates, and I guess the third element is confidence. What does the fourth element represent?
I originally thought that the fourth element was the index, but I came across a counterexample as shown below：
{"keypoints": [ [152.0, 0.0, 0.8896152973175049, 0.0], [172.0, 59.0, 0.9442344903945923, 1.0], [137.0, 64.0, 0.9105486273765564, 2.0], [122.0, 143.0, 0.9645923376083374, 3.0], [171.0, 182.0, 0.7813410758972168, 4.0], [211.0, 56.0, 0.9262291193008423, 5.0], [212.0, 138.0, 0.8982189297676086, 6.0], [191.0, 210.0, 0.8128728866577148, 7.0], [143.0, 194.0, 0.841970682144165, 9.0], [141.0, 316.0, 0.8266096711158752, 10.0], [140.0, 443.0, 0.8687180876731873, 11.0], [198.0, 194.0, 0.787242591381073, 12.0], [215.0, 316.0, 0.8372030258178711, 13.0], [239.0, 453.0, 0.8752306699752808, 14.0], [144.0, 0.0, 0.6165196299552917, 15.0], [163.0, 0.0, 0.6212925910949707, 16.0], [141.0, 0.0, 0.13327936828136444, 17.0], [184.0, 0.0, 0.7300758361816406, 18.0] ]}
There is no point 8 here, but point 18 appears instead.