From The RadioReference Wiki
If the trunking system loses its control channel or has certain other failures, it is no longer able to operate in the trunking mode. So instead of going into a condition that stops all communication, the system enters Failsoft. In this state all transmitters (channels) turn on and operate in a 'conventional' repeater mode. The subscriber radios are able to recognize this state and switch to a predetermined frequency (one of the trunk system frequencies, but not the control channel frequency) depending on their selected talkgroup. In most systems several talkgroups will share a frequency. Some talkgroups may not be assigned a failsoft frequency and these talkgroups will cease to operate during the failsoft period. If a particular failsoft frequency has also failed, the talkgroups assigned to that frequency will also be off the air during failsoft.