While most courses are usually taken for 3 credits, some (in particular seminars) can be taken for fewer credits, all the way down to 0 credits. The syllabus for the course should specify the workload per credit.
Here is a typical example for a seminar, but this can vary a lot across courses and instructors.
Here is an example for an introductory course, again with the caveat that this is just one of many different ways the workload may be distributed.
You should always discuss your course selection with your academic advisor, and this includes whether you should take a course for less than 3 credits. It may be tempting to take 3 seminars for 2 credits each instead of 2 seminars for 3 credits as that will reduce the number of term papers you have to write. But a PhD is not about gaming the system, it is about doing the work that will help you achieve your career goals. One of the main advantages of a seminar is that you get to write a term paper under the instructor’s guidance, a term paper that may well turn into a peer-reviewed publication down the line. Don’t optimize your course selection towards minimum effort, optimize it towards maximum pay-off.
G3 and G4 students who want to audit an LIN course should enroll for 0 credits. There are many reasons for that, but here are the ones that matter for you: