Short answer:
It is income that will show up in the employees W-2 and is taxable.

So if you fly in coach there is no charge but as an Enrolled Friend the employee will have the imputed income added to his/her salary on their W-2. If you fly in F/C or BF the employee will have the service charge deducted from their pay check and the imputed income added to the W-2.

Long answer:
Imputed income is reported on Form W-2. Imputed income is not subject to the federal income tax withholding rules. Employees may choose to have federal income tax withheld on the imputed income or pay what may be due when filing their federal income tax return. Tax penalties may apply if the employee has not withheld enough federal income taxes on the imputed income. Imputed income is subject to withholding for FICA tax purposes. Imputed income is also often relevant to determinations of support payments in divorce.