This has been nicely explained in a friends blog. Below is the description.

EAI Queuing is one of the Siebel integration tool to capture the SiebelMessage in a Integration Workflow in case the Workflow Errors out.

For more details, follow below link as I have already posted redarding this there -
http://www.siebelintegration.com/SiebelInterviewForum/index.php?topic=8.msg8#new
Post your comments in case you find any difficulty in getting it.