In the supermarket I saw a packet of cooking dark chocolate and a packet of dark chocolate slab. I want to ask are they same ? Is there any difference between ccoking dark chocolate and simple dark chocolate.
In many cases, yes. The specific case would depend on what, exactly, the "simple dark chocolate" had for ingredients, and also what the baking chocolate had for ingredients.
Most (but definitely not all) chocolate sold as baking chocolate has no sugar at all in it. This is what most recipes expect, unless they specify one that does.
Most (but perhaps not all) "simple dark chocolate" sold for eating has sugar in it, even if it's a "high-percentage" dark chocolate bar.
They can be substituted, but you should adust for the amount of sugar (more chocolate, since the sugar in it means there's less chocolate in it than chocolate with no sugar, and reduce the sugar in the recipe.)
I don't know if this is a difference between countries, but every single cooking chocolate from a supermarket I use contains sugar. I'm also used to recipes stating a minimum cocoa content if it really matters, and tend to prefer UK recipes, especially in baking– Chris HJan 9, 2018 at 21:08