How do logicians distinguish between formal and nonformal features of an argument?

Logicians distinguish between the formal and nonformal features of an argument by focusing on whether the validity of the inference depends on the argument's structure or its specific subject matter.1

Formal Features

Formal logic abstracts from the content of propositions to identify the underlying logical form or structure. An argument is considered deductively valid if its structure guarantees that the conclusion must be true whenever the premises are true, regardless of the specific content. Logicians use symbolic notation and variables to represent these structures, allowing them to test for validity independently of the truth or falsity of the premises. When an argument fails due to its structure—such as through an improper process of inference—it is classified as a formal fallacy.21345

Nonformal Features

Nonformal features relate to the content and wording of an argument, which are typically excluded from the study of formal logic. Arguments may be considered defective or fallacious for reasons other than their structure:21
* Material fallacies: These involve a misstatement of facts or errors in the argument's content.3
* Verbal fallacies: These arise from the incorrect or ambiguous use of terms.3

While formal logic is concerned strictly with the correctness of the inference form, the determination of the truth of the premises themselves is left to common observation or other specialized disciplines.1