When you create trade agreements through the price discount journals you might notice a flag on the lines called disregard lead time. If you are using delivery date control and using sales lead time you could choose to setup the lead time in the trade agreement. You do have the ability to override the sales lead time at an item default order or site specific settings but being able to set it in the trade agreement gives you the ability to have a setting for a group of customers or for a specific customer. This is a quick walk through the effect of the disregard lead time flag.
If you wanted to have a general look at sales lead time you can see this earlier post.