Reference frames are 2 specific keyframes that will be used to bootstrap the camera solving. They should therefore be rock-solid ones with good parallax. MatchMover chooses the reference frames automatically when you run the 3D tracker.
The data from 2D tracking is used to choose them, therefore if you change the data (e.g., you create a new point track), MatchMover may automatically choose two new reference frames.
MatchMover examines the image sequence and compares frames using two methods.
Method 1. Where all the intrinsic parameters are known (through camera information and constraints), it checks that there are either:
Method 2. Where all the parameters are unknown (except nonlinear distortion, which must have an approximately value), it checks that there are at least six known 3D points between two frames.
Ideally, they should include views of the same scene from a different position, which induces parallax between the images.
The automatically generated reference frames, if not locked, are recomputed when you select 3DTracking > Solve for Camera (F9) or when you use 3D tracking > Select keyframes.
They are locked either
when the user edited them, or when the lock icon at the right end
of the Track View Graded Ruler graduation
window is toggled. When locked, all the keys and references frames
are grayed, 
otherwise
they are highlighted in yellow, 
.
