Duration of Status
In certain visa categories such as diplomats, students and exchange visitors, the alien may be admitted into the U.S. for as long as the person is still doing the activity for which the visa was issued, rather than being admitted until a specific departure dates. This is called admission for "duration of status". For students, the time during which a student is in a full course of study plus any authorized practical training, and following that, authorized time to depart the country, is duration of status. The length of time depends upon the course of study. For an undergraduate degree this is commonly four years (eight semesters). Normally the immigration officer gives a student permission to stay in the U.S. for "duration of status." Duration of Status (or D/S) is recorded on Form I-94, Arrival-Departure Record. The Department of Homeland Security U.S immigration inspector at port-of-entry gives foreign visitors (all non-U.S citizens) an Arrival-Departure Record, (a small white card) when they enter the United States. Recorded on this card is the visa classification and the authorized period of stay in the U.S. This is either recorded as a date or the entry or D/S, meaning duration of status. The I-94 is a very important card to make sure you keep, because it shows the length of time you are permitted and authorized by the Department of Homeland Security to stay in the U.S.
Source: U.S. Department of State.