Can I start training from YOLOV4 weights?

[hweesiang]

I read that one should start training from yolov4.conv.137 pre-trained weights file.

1. May I ask what a pre-trained weights file is? Is it a "standard template file" for training, but does not contain any trained class?

2. Is it possible to use the yolov4.weights to start training, if I want to detect the original 80 classes of the coco dataset plus another 2 additional classes? Or does training strictly have to start from a pre-trained weights file?

3. Is it possible to only detect say 3 classes of the 80 classes of the trained coco dataset? If I try to change the number of classes from the original 80 to a lower number, I get an error "filters= in the [convolutional]-layer doesn't correspond to classes".