A higher amount of cocoa solids indicates more bitterness. Semisweet chocolate is dark chocolate with low sugar content. Bittersweet chocolate is chocolate liquor to which some sugar (typically a third), more cocoa butter and vanilla are added.
More cocoa solids in chocolate mean a stronger bitter taste. Semisweet chocolate is dark chocolate but has less sugar than other types of dark chocolate. Bittersweet chocolate is made by adding sugar, more cocoa butter, and vanilla to chocolate liquor.