If a state does not participate (or is not certified by the Department of Labor as meeting Federal requirements): (i) the Federal unemployment tax is levied within the state at its maximum rate, (ii) the system does not pay any unemployment compensation benefits within the state, and (iii) the Federal Government provides no grants to state governments to pay for the costs of administration.
If a state does not participate it is possible that the Federal unemployment tax is levied within the state.