The axis constraint buttons and flyouts appear on the Axis Constraints toolbar. See Using the Axis Constraints.
While the Axis Constraints toolbar is displayed, its controls update interactively to correspond to the portion of the transform gizmo that you are using currently.
Interface
Note: The default user interface does not show this toolbar. To display it, right-click an empty portion of any toolbar and choose Axis Constraints from the menu.
Restrict to X
Restrict to Y
Restrict to Z
Restrict to Plane flyout
Restrict to XY Plane
Restrict to YZ Plane
Restrict to ZX Plane
