There are two parts to CM 3215 Transport Lab, a lecture (section 0A) and labs (sections L01, L02, etc.). If you do not enroll in both the lecture and lab sections then you will receive a link error.
There are a number of classes that have multiple parts (a lecture, recitation, and/or lab), and you must enroll in all of the parts to register for the class. If you only register for one part you will get a registration error that says “Linked Course Required”. If you look at the section column when you search for classes (labeled “Sec”), the lecture sections are lettered (0A, 0B, etc.), the recitation sections start with an R (R01, R02, etc.), and the lab sections start with an L (L01, L02, etc.). All of the parts of a single course will have the same course number.
For example to enroll in CM 3215, Transport Lab, you must register for both the lecture and lab portions:
CM 3215 0A (lecture)
CM 3215 L01 or L02 or etc. (lab)
Classes with multiple parts include: MA 1160, MA 1161, MA 2160, MA 3160, CM 3215, CM 3310, CM 4110, CM 4120, and CM 4855.
Some classes instead require a “co-requisite”. This means that you must sign up for both classes (different course numbers) or you will get a “Linked Course Required” error. Classes that require co-requisites include: CH 1150/CH 1151, CH 1160/CH 1161, and MA 2321/MA 3521.