The difference is...

That some people release genetic material in their various bodily fluids {Secretor, roughly 80% of population is}, and other people don't {Non-Secretor}. The test involves a swab of your saliva then sent into the appropriate centre, various places can do this these days at reasonable cost.

As to the BTD, there is subtle differences in foods and disease susceptibility depending on your secretor-status.

As for me, when the Blood-Type Diet thing started, secretor status wasn't really known and the BTD made me feel worse, but it seemed a good idea...

Several years ago BTD was modified to include this variant - and it has worked marvelously for me in terms of mental clarity and a feeling of well-being. I wouldn't necessarily say its a 'diet' in the normal sense of losing weight.
For starters peruse the book: "Live Right 4 Your Type", by Peter D'Adamo
or the several books particular to your own Blood-Type {as long as they include info about Secretor-Status}