@ Slo Moe
TIBA is below Class G - more correctly, it isn't anything at all! There is no control service provided, nor any traffic information. The frequency is not monitored in any way, shape, or form by ATC. If you have an engine die and you declare a mayday, there is no-one (other than other aircraft which may or may not be there) to call help for you. You broadcast your position and intentions on the frequency, and try to seperate yourself from other traffic as best you can.
It is supposed to be a contingency for when ATC is unable to be provided, however it is being used more and more frequently in Australia due to an inability by Airservices Australia (the ATC provider) to staff sectors (I am just a pilot - I am in no position to say what the underlying reasons for this is) . We are talking center frequencies comprising what is normally Class A airspace with jets flying around at high speeds, 15nmi of seperation is nowhere near safe given the circumstances! Ideally it shouldn't be allowed to occur at all!