The Tab Order palette displays a numbered list of all the objects on the form, where each number represents the position of the object within the tabbing order.
Livecycle palette taborder Tab Order palette
The letter M inside a green circle indicates master page objects (visible only when viewing the form on the Design View tab).
A lock icon beside the first object on the page indicates that the object cannot be moved within the order (visible only when viewing the form on the Master Pages tab).
Livecycle palette masterlock Tab Order palette
See also 

Tab Order palette