UPDATE: ESPN.com's Marc Stein reports the Cavs, Celtics and Nets have agreed to a three-team trade that will open a max salary slot for LeBron James. To help get the deal done the Celtics will use the Pierce trade exception.
Whenever teams over the cap make trades and don't use all of an outgoing players salary to accept an incoming player the result is the generating of a traded player exception. This exception lasts for one-year and remains available as long as the team stays above the salary cap. Each exception can take back a player(s) for up to the amount of the unused salary plus $100,000.