A context tab is a hidden ribbon tab that is displayed when a corresponding element (such as an image or a table) is selected. One element can correspond to several context tabs. Therefore, the tabs are combined into categories. The collection of categories is accessible by the RibbonContextTabCategories property.
By default, HtmlEditor provides a Layout context tab that contains table-related commands and is displayed when a selection is in a table.
You can create custom context tabs and manually change tab visibility based on a selected element using the SetContextTabCategoryVisible method.
In this demo, a default Layout and custom Format tabs are provided for HtmlEditor.