Note
This topic is about classic commands.
There is a new way to define commands. See Modern commanding overview (preview).
Tab display rules control whether a specific tab is displayed for a ribbon or not.
Unlike other ribbon elements like groups or specific controls, you must explicitly provide a tab display rule for a tab to be displayed in the ribbon. By default, other ribbon elements will always display unless a display rule removes them.
<TabDisplayRule> elements require that the TabCommand parameter matches a <Tab> Command value.
In the RibbonDiffXml, tabs can be defined for specific tables or defined globally. If the tab is defined for a table, the only valid type of rule is <EntityRule>. Because defining a tab in the scope of a particular table already limits the tab to only that table, the only valid parameters are AppliesTo (PrimaryEntity or SelectedEntity) and Context (Form, HomePageGrid, SubGridStandard, or SubGridAssociated).
When you define a tab display rule globally in the RibbonDiffXml for the application ribbons, you can apply both EntityRule elements and <PageRule> elements.
See also
Customize commands and the ribbon
Define scaling for ribbon elements
Pass parameters to a URL by using the ribbon