There are two major types of Avoids: Temporary and Permanent.
The Temporary Avoid happens when you press the Avoid button once while listening to an active channel. That Avoids the channel for the current scanning session. When you power down the scanner, all Temporary Avoids are released and the channels become available when the scanner is powered up.
The Permanent Avoids are those that occur when you press the Avoid button two times in rapid succession while on an active channel. Another way is to set the Avoid in the Sentinel software or via a menu keypress sequence. Unavoiding these Avoided channels is as described above.
There is a third type of "Avoid" that might not be officially called an Avoid, but is functionally a very similar thing. That kind of Avoid is Location invoked. That happens when "stuff" (systems, sites, departments) are turned off due to location control. So if you are in Los Angeles, the New York stuff is avoided. The only way to clear this is, as you might image, is going to New York.