I am trying to follow 6.1 model, but "784" and "28 * 28" works totally different.

[Feiyang-umich]

model_01 = FashionMNISTv1(784,10,len(class_names)).to(device) works the same as the tutorial
while model_01 = FashionMNISTv1(28*28,10,len(class_names)).to(device) only gives ~10% percent accuracy with loss ~ 2.3.

Any difference between these styles?

[LuluW8071]

There is no difference, its basically same. As its just simpler way of readability for $28$ * $28$. Easier to know its $784$ comes from height * width.