This rule identifies: (1) the classification under subpart 4 for areas currently designated nonattainment for the 1997 and 2006 PM2.5 standards; (2) the deadline for states to submit attainment-related SIP elements for these nonattainment areas that are required pursuant to subpart 4; and (3) EPA guidance that is currently available regarding subpart 4 requirements. In doing so, this rule helps clarify deadlines and expectations following the DC Circuit opinion from January 2013, which held that EPA erred in implementing the 1997 PM2.5 NAAQS pursuant solely to the general implementation provisions of the CAA, rather than also the particulate-matter-specific provisions of subpart 4 of part D. In classifying all nonattainment areas as "moderate" under subpart 4, the EPA is affirming the outcome required "by operation of law" under subpart 4. To the extent one could argue that states' implementation plans demonstrating attainment of the PM2.5 NAAQS are overdue, this rule relieves burdens from the states and provides them an opportunity to address the interpretation announced by the Court earlier this year. Finally, the EPA directs states to long-standing general guidance that will help them ascertain the impact of subpart 4 requirements on their ongoing efforts to meet the 1997 and 2006 PM2.5 standards.