A TOEFL listening section textbook gives me a phrase such as below

Professor

>Another option for you this term would be to audit a course or two -- you wouldn't have to pay regular fees and full tuition for it.

Student

>Interesting. I never thought about sitting in on a course.

After googling the word at the title, [a page][1] describing this situation appeared.

Saying,

>Usually, in the Eastern USA, in the universities that I know, "sitting in on" means being in a class that one is not taking for a grade. The regular students take the class, they sit in it. A visitor (a student not enrolled in the class, or a visitor - a friend or relative of one of the students taking the class) may sit in on it, and listen to the lecture. I'm not sure if regularly attending the class without taking it - like for example a teaching assistant attending the class to see what exactly the professor is teaching - qualifies as "sitting in on" it.

Is the description of the site correct?

Thank you for your supports in advance.


  [1]: https://www.englishforums.com/English/SitClassClass/mnncw/post.htm%E2%80%9DAbout%20the%20phrase%20--%20sitting%20in%20on%20--%22