I am looking for the difference between the two – dont both do the same job iei classify the articles and the access to them?
[SalesForce] Data Category VS Article Type
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From Articles
The article record is the parent container of all versions of an article, whatever the publishing status (draft, published, archived) and the language. While KnowledgeArticle and KnowledgeArticleVersion represent any article in the Knowledge Base, __ka and __kav are the concrete representation of respectively KnowledgeArticle (“__ka” suffix) and KnowledgeArticleVersion (“__kav” suffix) for a specific article type. For example, Offer__kav gives access to every Offer articles. KnowledgeArticle and __ka give access to an article independent of its version. KnowledgeArticleVersion and __kav enables you to retrieve a specific article version and all its standard (KnowledgeArticleVersion) and custom (__kav) fields.
Article Type__ka
Gives access to an article from a specific article type independent of its version. This object is read–only and can't be used in a SOQL clause or in a WITH DATA CATEGORY DataCategorySpec SOSL clause. For more information, see KnowledgeArticle.
KnowledgeArticle
Gives access to an article independent of its version. This object is read–only and can't be used in a SOQL clause or in a WITH DATA CATEGORY DataCategorySpec SOSL clause.
From Inside Salesforce Knowledge (1): Objects / Data Model
At the API level, that capability of allowing multiple versions of an article at the same time induces a dual object structure. For example for an article type FAQ, you have two objects available in the API, FAQ_ka and FAQ_kav***, the former being mostly just a container, or a stable pointer to the article, when the later represents each version with their content. Those objects, allow access to only FAQ article types and all article types at once.
The word inherit, that is underlined above has its importance. All article types inherit from a common entity that is used for cross article types access. That entity is represented by the (dual) object, KnowledgeArticle and KnowledgeArticleVersion.
To the contrary of what you are doing with __ka" or "__kav" objects, which allows accessing records of one article type, including custom fields, KnowledgeArticle and KnowledgeArticleVersion objects allow you to query the entire knowledge base at once, across all article types, but do not give access to custom fields.
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Best Answer
I will take a stab at a non technical way to think about it. Article types are used to define different kinds of articles, from a simple FAQ to a complex procedure including the fields available and layouts to use per channel the article might be viewed in. As you rightly noted you can control visibility of article types at a user profile level or via permission sets.
When you search the knowledge base data categories come into play. search results can be refined to more relevant results quickly using data categories. For example, if you had 10000 faq articles on Italy, your profile might have access to those 10000 articles because your user profile permits you to see faq's. Using data categories at an article level, you attribute categories of 'north' (geography data category), 'accommodation' ( type data category) and 'summer' (season data category) to a particular article. If you filtered the faqs by one or more of these categories you'd quickly zero in on the relevant article for summer accommodation in northern italy.
Making more sense ?