TActionLink descendants call IsCheckedLinked to determine whether they need to propagate changes to the Checked property of the action so that they are reflected by the client object. When IsCheckedLinked returns true, the action controls the Checked property: Changes to the Checked property of the action are reflected by changes to the corresponding property of the client object. When IsCheckedLinked returns false, the client object does not reflect the Checked property of its action, either because it does not have a corresponding property, or, more commonly, because it manages the value of its corresponding property independently. 

As implemented in TActionLink, IsCheckedLinked merely checks that the Action property specifies a descendant of TCustomAction (that is, it is an action that has a Checked property). Descendant classes override IsCheckedLinked to determine whether the Checked property is linked with the appropriate client property. 

Checked 

SetChecked 

Action