In an unclear (unclear) sentence, the reader cannot understand the message the writer wants to convey. The error is global (more serious) because it affects more than a clause and may affect whole parts of a text.
An unclear message may or may not be related to problems with grammar in a sentence.
Unclear sentences like:
- Pressure is one of equipment to bring up our skill by ourself
- I can experienced the truth of studying by a coffee are grammatically incorrect and have an unclear message.
On the other hand, unclear sentences like
- Waiting for these negatives to grow is not a particular way
 unclear = grading symbol for unclear sentences