The TrackClicks system preference is documented in the Manual at https://koha-community.org/manual/latest/en/html/opacpreferences.html#trackclicks It says: "By setting this preference to one of the track options you will allow Koha to track every link clicked in Koha." This is incorrect: when testing I found that only clicks on the records' 856$u or item URI are tracked. The manual should be updated.
Have updated the description. As far as I could tell, MARC21 is supported (856$u and 952$u), but UNIMARC isn't fully supported (would require changes to the OPAC XSLT).