Categories can be applied to both tasks and faqs. To restrict the use of a category to only tasks, select  tasks. To restrict to only faqs, select  faqs. Otherwise, select  any

Since categories are hierarchical, you can accidentally (or on purpose) mask subcategories by giving them incompatible datasets. For example, if you have category Furniture, which is only applied to faqs, and you create a subcategory Chairs, which is only applied to tasks, then you will never see the Chairs subcategory in any of the task drop-downs.