Select of Keyframe (Question)

[mokhtarbouain]

Hi all,
Thank you for the great book.
I have a question please, about the selection of Keyframe:
In the page 303, I found the following statement:
If the tracked features are fewer than a threshold, we set the current frame as a keyframe.
i.e. Two consecutive keyframes have to involve sufficient visual change? It is the practice to select or choose the Keyframe?
So, after that, do we have to triangulate all the points (or features) in Keyframe to create the Landmark of map?
Thank you
@gaoxiang12