I think the first refers to rules that directly discriminate against imported goods and favor domestically made products. And the second refers to indirect hinderance for imported goods, even if the rules apply equally to both domestic and imported goods.
The Keck case refers to the French law, which prohibited companies from selling goods at a lower price, than their purchased price. But the same law didn't prohibit manufacturers of goods from selling their products at a loss.
This meant that French retail companies couldn't purchase goods from other EU countries and sell them at a loss in France. But French manufacturers of such goods could make them in France and sell them at a loss. This gave an unfair advantage to French-made goods in France.
This law didn't directly discriminate against imported goods. But in a price-war type of situation, French-made goods had an advantage because of this law. The EU court decided that this situation wasn't a violation of Article 34.
But later cases in other EU countries have reversed this precedent and included rules for selling arrangements as being covered by Article 34.
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