18. The system of claim 17 , wherein determining that content items for only a first of the two identified content categories will be presented in the particular module type comprises: determining that presenting both of the of the two identified content categories in the particular module type will cause a number of the knowledge modules of the particular module type to exceed a maximum number of knowledge modules that are allowed to be of the particular module type; and determining that the content items in the first content category have a higher rank score than the rank score for the content items in the second content category.
18. The system of claim 17 , wherein determining that content items for only a first of the two identified content categories will be presented in the particular module type comprises: determining that presenting both of the of the two identified content categories in the particular module type will cause a number of the knowledge modules of the particular module type to exceed a maximum number of knowledge modules that are allowed to be of the particular module type; and determining that the content items in the first content category have a higher rank score than the rank score for the content items in the second content category. 19. The system of claim 18 , wherein the one or more processors are further configured to perform operations comprising selecting the content items in the first content category for presentation in the particular module type, the selection being in response to determining that the rank score for the content items in the first content category is greater than the rank score for the content items in the second content category.