Can varifocal loss be applied to softmax classifier?

[Icecream-blue-sky]

Actually, I don't know why most one stage detectors use sigmoid classifier even though COCO is a multi-classification task. Do you have a more reasonable explanation？I'm really confused about it. And could FCOS-based detector use softmax classifier? I'm applying FCOS-based detector(similar to VarifocalNet) to logo detection ( the num of categories could be 1000). But I get many false positives at the same position(with different categories). I think this might be the reason for using the sigmoid classifier(instead of softmax classifier).

So I want to use softmax classifier in FCOS, but I'm worried that the varifocal loss can only work on sigmoid classifier, is that so?