Here’s how the law in question (the Military Commissions Act of 2006) defines "Persons Subject to Military Commissions":
`Any alien unlawful enemy combatant is subject to trial by military commission under this chapter.
And here’s how the act defines "alien unlawful enemy combatant":
(1) UNLAWFUL ENEMY COMBATANT- (A) The term `unlawful enemy combatant’ means–
`(i) a person who has engaged in hostilities or who has purposefully and materially supported hostilities against the United States or its co-belligerents who is not a lawful enemy combatant (including a person who is part of the Taliban, al Qaeda, or associated forces); or
`(ii) a person who, before, on, or after the date of the enactment of the Military Commissions Act of 2006, has been determined to be an unlawful enemy combatant by a Combatant Status Review Tribunal or another competent tribunal established under the authority of the President or the Secretary of Defense.
(3) ALIEN- The term `alien’ means a person who is not a citizen of the United States.
So as I read that, it means that anyone who isn’t a US citizen (including foreigners here on valid work permits) who is designated as a Bad Guy by "another competent tribunal established under the authority of the President" can be thrown into a secret prison.