What does tax debt have to do with a passport?

Passports are issued by the State Dept., not the IRS. But a 2015 law requires the IRS to notify the State Dept. of taxpayers having “seriously delinquent tax debt,” generally defined as tax debt exceeding $50,000 and for which a lien has been filed (for tax years beginning after Jan. 1, 2016). Unless exceptions apply, such tax debt is grounds for denial of a passport or revocation or limitation of an existing passport. The IRS recently issued guidance for the implementation of this provision, in Notice 2018-1.