Two kinds of difficulty should be distinguished.
Verbal difficulty may arise from unfamiliar words or unusually hard syntactic arrangement.
Conceptual difficulty arises when the intended meaning remains obscure even after the words have been understood.
QuestionMain concern
Can the words be understood?Diction and construction
How do they indicate the intended sense?Meaning and contextual indication
The two may overlap, but the introduction particularly associates the second discipline with avoiding conceptual obscurity.