May be used when a call exceeds 3 of the following 4 criteria:                                                                              

An event/incident involving response from four (4) or more agencies
An event/incident involving a duration of at least six (6) or more hours
An event/incident involving response from at least three (3) levels of government
An event/incident where normal use of common simplex (local talk-a-round) channels
will not support the incident commanders needs

Several V-TAC frequencies are also used in Maine. Please see the national Radio Reference interoperability page for a listing of those.