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"Pancerz" can be used to describe different things:
- a part of an armour or a shielding in general (for example: of a tank),
- a hard layer covering (and protecting) an animal (compare with "pancernik" = "armadillo")
- someone's attitude (if he/she is hiding his/her fillings)
- perhaps something else...
"Zbroja" is mainly used to refer to an armour of a knight.