Recently category theory started to appear more frequently in my paper readings. From the Wikipedia page [Timeline of category theory and related mathematics][1] it seemed that category theory had applications in string theory in various places. For example, it showed up in discussions concerning topological structures. 

However, unlike group theory and set theory, category theory was not usually offered in standard lectures, and less so for mathematical physics. The [Wikipedia page][2] explained the entities of category theory (objects, morphisms, and binary operation), which was understandable, but the webpage did not provide the examples of applications. 

There are some useful references for category theory from the math exchange:

1. https://math.stackexchange.com/questions/2630894/recent-sets-of-notes-newly-available-online-books-on-category-theory

2. https://physics.stackexchange.com/questions/246208/implementing-category-theory-in-general-relativity

3. https://math.stackexchange.com/questions/370/good-books-and-lecture-notes-about-category-theory?rq=1

but it's unclear to me how category theory is frequently used in the string theory, i.e. in abstract algebra, there's Lie groups, generators etc. Set theory is even more commonly accepted as a standard tool, but why are categories so useful? Is there a "special string category" that's particularly useful in string theory? 

How is category theory implemented in the string theory, and are there some references or lecture notes for category theory in string theory?

  [1]: https://en.wikipedia.org/wiki/Timeline_of_category_theory_and_related_mathematics
  [2]: https://en.wikipedia.org/wiki/Category_theory