On the FAA domestic flight plan form (FAA Form 7233-1) the equipment code is a single character placed in block 3 (Aircraft Type / Special Equipment) as a suffix to the aircraft type code.
The FAA began requiring the ICAO format form for domestic flights desiring RNAV routes on 29 July 2008.
The ICAO format has already been in use for all domestic flight plans in Canada, Mexico and many other countries for a number of years.
To see the differences in the coding systems, consider a VFR aircraft with a VHF communication radio, VOR receiver with glideslope for ILS approaches, ADF, a GPS and a pressure altitude reporting transponder.
More letters with the ICAO format mean more information about the aircraft's radio navigation capability is available to the ATS controller than with the older FAA coding system.