Hi Xiao Ping,
I have read about this in Ed Romans
EJB 3rd Edition book. Please refer to page number 138 - 139 in 3rd edition of the book. There he also explains about how it is being used in EJB Lifecycle as well. Hope this will clear out your confusion.
After going through this, if you are not clear, post your question will try to help you.
Thanks
Shibu