50% dark chocolate refers to the percentage of cacao mass, cocoa butter and cocoa solids in dark chocolate which is mixed with 50% of sugar and other ingredients.
With 50% of chocolate and 50% of other ingredients, this chocolate is comparitively sweet and have a rich and dark flavor.
Cocoa solids are the main ingredient for 50% dark chocolate which is mixed with sugar, cocoa butter and sometimes added flavor like vanilla.
Chocolate is made from the seeds of Cacao fruit which is fermented, dried and roasted then hulled and turned to a paste.
The chocolate paste is then refined and tempered then molded to bars and frozen solid for selling.