Tier refers to the scope of a TMI. 1ST TIER refers to those enroute centers immediately adjacent to the one where a TMI or a constraint in place. For example, if there is a 1ST TIER ground stop in place for IAD, which is in Washington Center (ZDC), that would mean that all traffic departing to IAD from ZDC, Boston Center (ZBW), New York Center (ZNY [..]