Lossless ADIF

While ADIF is a standard for Logbook data exchange, the contents are not.  Some ADIF files may or may not contain certain fields depending on the capability of the programs that created them.   Some programs store more data in ADIF than others.  Therefore, two logbooks that represent the same set of QSOs might have vastly different sizes depending on what data was included in them.   Most of the ADIF fields are, after all, optional.

This gives rise to an issue whereby you may have an ADIF file that contains the maximum number of data fields but when you upload it to a logging program, some of those fields are discarded.   Later, an export of the same data from the logging program will be missing the discarded data.

QRZ stores everything you upload to it, even if we don't use or display it.  Then, if you subsequently Export your data from QRZ, those unused data fields will be included in the output (unless you've selected something else).

This technique of saving information we don't use is called "Lossless ADIF."