An axis that uses the world coordinates will be placed through the start and end points of the solved camera animation. An axis that can be edited interactively in the Viewport will not be displayed. This mode is only designed for use with camera tracking.
This is the more commonly used option, which ascertains the wold or Object Tracker coordinate axes based on the vector aligned to the features.
Use this setting to define the coordinates axis that the vector should represent. The vector will be displayed with the respective axis color in the Viewport.
If None is selected, the axis will remain undefined.
Enable this option to reverse the axis, for example, if you have defined the Y axis and the camera or Object Tracker is upside-down.
The Length setting scales the vertex cluster to a realistic size. Estimate how long the vector would be in reality. In the above example you would match the vector for both (user) Tracks to the front edge of the keyboard. The keyboard is 43 cm wide, which is the value you would enter in the numeric field to the right of the Length setting, and the vertex cluster will be scaled accordingly so that both features associated with the Tracks will lie exactly 43 cm apart.
Drag the features into these fields that should serve as start and endpoints for the vector. Normally, these will be set automatically when the vector is positioned interactively in the Viewport.