In that latter situation, a transferring student may discover they cannot graduate until they take courses at the second institution which partially overlap or repeat material they have previously studied at the first one.
Course articulation may be done on an ad hoc basis when a student actually wishes to transfer.
It may also be done pursuant to existing course-to-course comparison data, or based on formal articulation agreements.
Their consensus is then formalized in a written agreement which is used by students and advisors and is regularly updated according to a mutual schedule.
While both might cover approximately the same material by the end of one academic year, they may tackle subtopics in a different order, meaning that a student who transfers without taking the entire sequence at one institution will have significant knowledge gaps.