One famous example is de Saussure's Cours de linguistique générale, which was published from the notes of Charles Balley amd Albert Sechehaye. A citical edition was pnlished far later.
The idea is to see how course notes reflect a teacher's pedagogical practices and, as such, can be used as data to analyze said teacher"s pedagogical choices. Let's say a teacher claims to be using Communicative Language Teaching, I'd like to look at their students' notes to see if their reported practices (what they say they are doing) matches what they are effectively doing. The students' course notes seem like valuable data in that respect.
A theoretical article, in general, comprises or alludes to new or established abstract concepts relating to a certain field of knowledge. These articles are peer-reviewed, however, they do not often include research or experimental data.