In a normal briefing cycle, the movant (here the government) files an opening brief, then the respondent (ACLU) files an opposition, and the movant gets to file a second brief in reply. A sur-reply requires permission, and allows the respondent to challenge statements in the movant's reply brief.

Thanks!

In other words, it's: Gov - Brief supporting motion to void injunction ACLU - Brief in opposition to voiding injunction Gov - Reply Brief in support of voiding ACLU - Sur-Reply Brief* in opposition *Filing a sur-reply requires leave of court, which is often granted if the reply raised new issues.