"TT" code used in the third field in batch input formats (in lieu of a US State code) to indicate that the following field is a Tribal Code. Also appears as the first field in a Correct form when in Tribal Mode. Note that it is possible to define sites in non-US states. There are currently sites in the AQS database that exist in Mexico (80) and Canada (CC).
More information on Batch load formats can be found in the AQS Data Coding Manual and AQS Data Input Formats at http://www.epa.gov/ttn/airs/airsaqs/manuals/manuals.htm