I have this video taken from behind a airplane propeller which is a bit shaky in the background.
The propeller occupies the left part of the clip.
If I apply the stabilize effect I clearly see the spinning propeller confusing the algorithm, resulting in a twitchy video.

Is there a way I can tell the stabilizer effect to only look at a given part of the clip to compute its transformations?

it's not possible to stabilize only a part of a video, stabilization mean moving x and y the image, zoom the image,and rotate the image automatically...
this need to be done on the entire image (the algorith can't distort the image to stabilize it)

if you want to keep centered an item on the screen, the motion capture effect is what you need. https://www.youtube.com/watch?v=2G_UfhDjm58

That makes sense and it was already clear to me.

I was asking if there is a way to make the effect use only a part of the clip to compute the transformations that of course must be applied to the whole clip.

That is: act like you only want to stabilize a part of the clip, and then apply the transformations (rotate, zoom, move) to the whole frame as usual

ok i understand now... but i think that right now is not possible because the stabilization is a Job.. and can't mixed with other effects.. too hard to mix the stabilize job with a motion tarcker to abtain something good (using for stabilization only the necessary part with a zoomed sub rendering) this reuqest need to be adressed to MLT developers