Flannel shirt is fine, but flannel pants are pajamas are only for wearing inside. A T-shirt made of jersey material is fine to be worn out, but thin jersey pants are pajamas and should only be worn at home, so says conventional wisdom.

But, if shirt is made of pant, then it’s nice/slightly fancy. Denim shirt? Nice, durable, suitable for a restaurant.

I don’t know if this applies to women’s fashion, as it seems to have very different rules to men’s wear