請問船務出口中的demurrage and detention,點解?

Demurrage fee 延滯費，The rental charge on containers. Goods already arrived in port but not collected within a free time. After this free time say, 3 days, after arrival, the container company will charge this demurrage fee, a warehouse charge.

Detention fee 滯留費，the rental charge on the empty container not yet returned.
There is a time limit say, 3 days, after the container was taken out for moving out the goods. And, this container when empty should be returned to the container company within the specified time or else a detention fee ( penalty )will be levied.

See these web-sites for a typical charges on these 2 issues

http://www.oocl.com/poland/eng/localinformation/ddfreetime/

http://www.maerskline.com/link/?page=lhp&path=/europe/poland/demurrage_detention