Hi Kwong,
The usual format is text with a tab delimiter. However, you do not need to concern yourself with the file type and layout. There are several software packages that can create a file in one of the BAA formats. These include MuniPack, ASTAP, AIJ or AIP4Win in combination with the BAA Photometry spreadsheet, and MetroPSF. This page has links to each of these software packages at the top of the page:
https://britastro.org/vssdb/notes_submissions.php
If you have a copy of MS Excel and are comfortable using it, then that might be a good option as I think you are already using AIJ. Otherwise ASTAP, MuniPack and MetroPSF are good options.
I can’t give detailed advice on the individual packages, though I am sure there are members of this forum who can give advice on choosing which to use and how to use it.
Best wishes,
Andy (BAA Photometry Database Manager)