STIL uses a few private conventions when writing and reading FITS files. These are not private in the sense that non-STIL code is prevented from cooperating with them, but STIL does not assume that other code, or FITS tables it encounters, will use these conventions. Instead, they offer (in some cases) added value for tables that were written by STIL and are subsequently re-read by STIL, while causing the minimum of trouble for non-STIL readers.