[fw-formats] Zend_Pdf meta data 'Trapped'

Hi,

I have read the manual and had a close look at the code but still I have
no idea what the 'Trapped' meta data within Zend_Pdf is used for. What
is the real purpse of 'Trapped'? Could some one please explain it in
other words for a fool like me? :-)

http://framework.zend.com/manual/en/zend.pdf.info.html

Thanks and best regards,

Ralf