The qualification is the process of checking for a host in order to know whether it is alive or not, and have that information reported in milliseconds with sip show settings.

You can set how often the qualification will be done with the ‘qualifyfreq’ setting in the sip.conf file. If  your Asterisk box if facing the Internet, and the device connecting to it is inside of a NAT device, you have to turn on qualify=yes to keep the NAT session opened. Also, qualifying devices is the way for Asterisk to keep track of who is available for things like BLF.