The Yolo format required params normalised wrt. the dimensions of the image.
So an object of class 3 with its center 20% of the way down from the top of the image and 30% of the way across from the left with a width equal to 10% of the image width and a height equal to 15% of the  image height would be labelled as:
3 0.3 0.2 0.1 0.15
See here for more description: https://github.com/AlexeyAB/Yolo_mark/issues/60 5