In a nutshell, whenever you trade a player with a higher salary than the one(s) you are receiving, you get a trade exception for the difference. You can use this exception to trade for another player within a year (if the player you're trading for has a higher salary than the allowed 115% + $100K). You cannot use these exceptions for signing free agents, or extending contracts to your own players - they're only useable for trades and expire within 1 year.
There's a lot more complicated rules than what I just posted, but that's it in a nutshell...
Jose