The Form41 data is all about the Financials, while T100 data is all about passengers.
The Diio Mi U.S. DOT Form 41 section provides relevant statistics on the operations of the major U.S. carriers. Key components of the Diio Mi Form 41 data are cost detail and revenue performance by airline including the following: load factor, yield, RASM, CASM, and a variety of other efficiency measures.
The Diio Mi U.S. DOT T-100 section is sourced from the U.S. Department of Transportation and records the number of passenger enplanements at a detailed level. The data records the number of passengers on board an aircraft (i.e. flight segment), however it does not record the passengers' actual O&D. The true passenger O&D can be obtained through the reports in the U.S. DOT O&D section.
In theory the ASMs should match between the two data sets, but in practice they won't. The difference occurs because carriers provide incomplete or inaccurate data directly to the DOT.
Carriers will sometimes file corrections with the DOT and revised figures will be released. When that happens, we will post a notice in our Data Updates > Recent Updates page indicating we have restated historical data.
T100 data is released monthly, around the 15th, and Form41 data is released quarterly, about 3 months after the end of the quarter being released. You can see the available date ranges in our Data Updates > Data Date Ranges page.