The target language may or may not be known in advance, though in most pedagogic EFL contexts, it will be.
Target language can also vary in its specificity. A vague description of target language may be "students will use level-appropriate language to complete the given task". A moderately specific description highlights specific target lexis/vocabulary and structures. A highly specific description identifies the only language permitted, for example a gapfill with given multiple choice answers.
Given that the process is likely to be imperfect, students may produce language that is similar to the target language.