From Xojo Documentation
The text of a secondary message.
This appears in a smaller font size below Message only on macOS. On other platforms it appears in the same font size as Message. Use this text to provide a fuller description of the situation, its consequences, and how to get out of it. For example, a warning that an action cannot be undone is an appropriate use of explanation text.
The following example creates and manages a "Save Changes" dialog box without the need to create an instance of the Window class.
Var b As MessageDialogButton // for handling the result
d.Icon = MessageDialog.GraphicCaution // display warning icon
d.ActionButton.Caption = "Save"
d.CancelButton.Visible = True // show the Cancel button
d.AlternateActionButton.Visible = True // show the "Don't Save" button
d.AlternateActionButton.Caption = "Don't Save"
d.Message = "Do you want to save changes to this document before closing?"
d.Explanation = "If you don't save, your changes will be lost. "
b = d.ShowModal // display the dialog
Select Case b // determine which button was pressed.
// user pressed Save
// user pressed Don't Save
// user pressed Cancel