Although not a court specialist, from TV court dramas, particularly in USA, I understand that if an "exception" is taken to a particular juror or to jurors and it is upheld by the court, that juror is excused from participating in that particular court case.
The "exception" woud be based on potential pre-existing bias of the juror(s) against (or in favour of) the accused before any evidence is heard either way.
I believe that there may be a limit on the number of "exceptions" that can be made (otherwise it would be impossible to chose a jury).
Hope this helps
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