From Teflpedia

Correctability is the degree to which an error can be corrected towards an acceptable norm.

If a teacher recognises a straightforward error, then they may correct that. However, students sometimes write or say things which are incomprehensible, or deviate so far from norms that the only way of correcting it would be to rewrite the entire sentence. Sentences may also be difficult or impossible to correct if information on the student’s intended context is lacking.